Durum Wheat Chemistry and Technology, Second Editon

CHAPTER 12 CHAPTER 1

Origin and Distribution of Durum Wheat Genetic Diversity in the World Alessandro Bozzini (Retired) Food and Agriculture Organization of the United Nations Rome, Italy

Jacques David Montpellier Supagro UMR Amelioration Génétique et Adaptation des Plantes Montpellier Cedex 2, France Vincenzo Natoli ISEA S.r.l.

Corridonia, Macerata, Italy

Evolution and Classification of Wheats A Common Ancestral Genome for Grasses All the wheats belong to the genus Triticum, a member of the grass (Gramineae or Poaceae) family. Barley (Hordeum vulgare L.) and rye (Secale cereale L.) belong to the same Hordeae tribe, in which one or more flowered spikelets are sessile and alternate on opposite sides of a rachis (the main axis of the inflorescence), forming a true spike. They are also close relatives of some weeds like Agropyron and other wild grasses that can be crossed with wheat (Thinopyrum, Leymus, Aegilops). This related group of Gramineae is often referred as the Triticeae, defined by its relatedness to wheat. Triticeae species are adapted to the steppes or semiarid areas, characterized, in the Northern Hemisphere, by winter rains and dry summers, where they develop with available fall-­w inter moisture and, depending on the elevation of these areas, reach maturity in late spring or summer (Van Slageren 1994). They grow in different ecological niches, some species being more adapted to dry and warm conditions (barley) and others thriving in more moist areas (Aegilops tauschii Coss.) or mountainous regions (wild rye). Most wild Triticeae species thrive in the eastern Mediterranean, Near East, and southwestern Asia, but some species can also be found in Australia (e.g., Australopyrum spp.), in western Mediterranean Europe, and the Maghreb region (Aegilops spp.). Wild Aegilops species (referred to as wild Triticum in some classifications) are closely related to wheat. They can cross with wheat either spontaneously or via controlled crosses and sometimes give rise to fertile offspring.

The Triticeae diploid species share a common number (seven pairs) of chromosomes, inherited from a common ancestor. Thus, even if evolutionary processes such as translocations (changes in gene order or gene content) occur, the derived homoeologous chromosomes still share large similarities among

the different Triticeae species. For instance, chromosome 1H of barley is homologous to the chromosome 1R of rye.

This ancestry takes even deeper root in the phylogeny of grasses, including rice, maize, sorghum, sugarcane, and millet, which are all important crops for human and animal nutrition.

Recent genomic evidence supports the hypothesis that all grass genomes evolved from a common ancestor with a basic number of five chromosomes through a series of whole genome and segmental duplications, chromosome fusions, and translocations (Salse et al 2008). Conservation of gene order within the Triticeae, which includes sets of common genes involved in the expression of similar traits, has permitted the use of DNA sequence data from barley or rice to help researchers understand the genetics in wheat (Salse and Feuillet 2007).

Polyploidization: A Common Evolutionary Feature in Triticeae The Triticum genus is complex and rich in species. A description of their characteristics and related genomes is presented in Morris and Sears (1967), Kimber and Sears (1987), and Bozzini (1988). However, for the purposes of this chapter, the taxonomic classification system of Van Slageren (1994) will be followed. A “Triticum Comparative Classification Table” appears at https:// www.ksu.edu/wgrc/Taxonomy/taxintro.html.

When species grow in the same area, spontaneous hybrids may be observed; many examples are reported in the herbaria (Van Slageren 1994). Such interspecific hybrids are usually highly sterile; the homeologous chromosomes of the two differentiated genomes do not pair uniformly during meiosis and produce nonviable unbalanced gametes. In some cases, though, meiosis alterations generate the formation of unreduced gametes (gametes carrying 2n = 14 chromosomes instead of the usual set

x

1

2

x

Durum Wheat: Chemistry and Technology, 2nd ed.

of n = 7 chromosomes). The mating of a 2n male gamete with a 2n female gamete may lead to a new stable and fertile polyploidy species (allotetraploid) that consists of 2n=4x=28 chromosomes (Kihara and Lilienfeld 1949, Xu and Dong 1992). This ability to generate unreduced gametes is genetically determined and also observed in cultivated wheats (Zhang et al 2007).

Spontaneous polyploidy is common in plants, and many combinations between diploid Triticeae genomes can be observed in nature (Van Slageren 1994). Interspecific allopolyploidization can also involve species with higher ploidy and can lead to hexaploidy. (Octoploidy and higher ploidy levels are rare in the Triticeae.)

In newly produced polyploids, the homeologous chromosomes might still pair (mimicking autopolyploidy), leading to abnormalities in gamete formation and subsequently reduced fitness. Pairing between the different genomes can lead to chromosome rearrangements. In T. turgidum subsp. dicoccoides, a wild allotetraploid wheat (Badaeva et al 2007), as in T. araraticum (T. timopheevi subsp. armeniacum) (Jiang and Gill 1994), high rates of diversity for chromosome rearrangements can be found between individuals, including reciprocal translocations and chromosome inversions. The further stabilization of allopolyploids requires a restriction of pairing between the homoeologous chromosomes (Cifuentes et al 2010). In polyploid wheats, mechanisms for repression of homeologous pairing are under genetic control (Okamoto 1957, Riley and Chapman 1958, Sears 1976). One of these genes with a major effect, Ph1 (pairing homeologous), has been recently identified at the molecular level (Griffiths et al 2006).

Polyploidy appears to have occurred spontaneously in the Triticeae tribe in different periods of history. Divergent diploid genomes have been combined to produce new polyploid species by spontaneous hybridization between diploid (or diploid with tetraploid) Triticum or Aegilops species. For example, out of the 22 Aegilops wild species classified according to Van Slageren (1994), only nine species, representing six divergent genomes, are diploid. Most of the remaining species are allotetraploid, and a few are allohexaploid (Van Slageren 1994). The fact that high ploidy levels are observed in Triticeae strongly supports the hypothesis that the introduction of genetic variability by multiple origins can increase the ecological amplitude and evolutionary success of allopolyploid species compared with their diploid progenitors, for adaptation both in the wild and in agriculture (Meimberg et al 2009).

Classification of Wheats Within a Polyploid Series In 1753, in “Species Plantarum,” Carl Linneus proposed the first classification system of plants, including wheats, based on morphological and physiological differences. In the twentieth century, as a result of pioneering cytogenetic work, the number of chromosomes present in each morphologically recognized type became an objective key for classification of wheats. The cytogenetic and cytological analysis showed that wheats fall into three basic natural groups, each one characterized by having in each somatic cell 14 chromosomes (seven pairs) or a multiple of 14 chromosomes. The groups are diploid wheats (e.g., Triticum monococcum subsp. monococcum, or einkorn, having 14 chro-

mosomes); tetraploids like T. turgidum subsp. durum, or durum wheat, having 28 chromosomes; and hexaploids like bread wheat (Triticum aestivum subsp. aestivum), having 42 chromosomes.

Both Aegilops and Triticum species are distributed within a polyploid series from the basic diploid number to a hexaploid state.

As all component genomes appear to share a common Triticeae ancestor with seven chromosome pairs, the chromosomes of polyploid wheats can be grouped into seven homeologous groups. Homeology is the state of coancestry between chromosomes present in the same polyploid species. A chromosome of a constituent genome does not normally pair at meiosis with its homeologous counterpart even if it is, at least partially, able to substitute for it. The genome content of a polyploid species can be elucidated by the study of meiosis in interspecific hybrid combinations involving either different wheats or wheat crossed with wild diploid Triticum or Aegilops species.

Hexaploids, like T. aestivum, combine the chromosomes of three basic genomes, which have been grouped according to their meiotic affinity and referred to with three letters to indicate their origin (see Fig. 1.1): bread wheat has 14 chromosomes of the A genome plus 14 chromosomes of the B genome plus 14 chromosomes of the D genome, equaling 42 chromosomes).

Tetraploid species combine two different homeologous genomes. A group of subspecies of T. turgidum (including durum wheat) have genomes very similar to the A and B genomes of hexaploid bread wheat, T. aestivum. Another tetraploid species, T. timopheevi, has an AAGG genomic formula. Its A genome is genetically close to that of T. aestivum, and the G genome is different but is related to the B genome of both tetraploid and hexaploid wheats.

Durum wheat is a subspecies of T. turgidum and is referred to herein as T. turgidum subsp. durum. It is currently the most cultivated tetraploid wheat (genomic formula AABB) of the Triticum genus. Researchers have analyzed the structure of the genomes in order to clarify the genetic mechanisms underlying their evolution and crossbreeding (Martinez-­Perez et al 2001).

Molecular markers allow a fine study of the gene content and order of the seven homeologous groups in wheat. Homeologous variation may consist of sequence divergence, evolution of gene copy number, and translocation. Polyploidy itself may lead to genomic modification; within the three copies available for each gene, some may have been eliminated rapidly (Akhunov et al 2003) or their regulation strongly modified (Levy and Feldman 2004).

After this basic discovery, a number of botanists and geneticists proposed a series of classifications. There are currently 13 such classifications, illustrating the controversies in taxonomic groupings. To follow the recommendations made at the Taxonomy Workshop held at the Ninth International Wheat Genetics Symposium in 1998, the GrainTax system has been proposed to establish synonymy tables. Readers are advised to consult the current compilation at the Wheat Classification Tables site (https://www.ksu.edu/wgrc/Taxonomy/taxintro.html).

Diploid Progenitors of Wild Polyploid Wheats The A genome of durum and bread wheats is closely related to the genome of the current wild diploid species T. urartu (Au genome). A sister wild species (T. monococcum subsp. aegilo­poides),

Origin and Distribution of Genetic Diversity or T. boeoticum, has also been classified as belonging to the “A” genome group with a slightly different genome (Ab). The wild T. boeoticum and T. urartu species are differentiated morphologically, ecologically (Zohary and Hopf 2000), and at sequence and molecular marker levels, but this latter divergence has been found to be weak (Dvorak et al 2006) compared to their divergence from other close Aegilops species. These two species, T. uratu and T. boeoticum, can be hybridized, but the F1 generation is mostly sterile (Johnson and Dhaliwal 1976) and their chromosomes do not recombine easily (Dubcovsky et al 1995). The cultivated diploid wheat T. monococcum (einkorn) shares the Ab genome of T. boeoticum (Brandolini et al 2006, Kilian et al 2007). The Ab genome of T. boeoticum is also very similar to the A genome of both the wild tetraploid T. timopheevi subsp. armeniacum (syn. T. araraticum) and T. timopheevi subsp. timopheevi (syn. T. timopheevi), suggesting that T. boeoticum is the diploid progenitor of the A genome of these tetraploid wheats (AAGG genomes).

The B genome of durum and aestivum wheats is related to the S genome of the current Aegilops speltoides species, a member of the Sitopsis group, even if evidence suggests that the ancestor of the real B genome of cultivated wheats may have differed significantly (Zohary and Feldman 1962, Blake et al 1999).

The S genome of Ae. speltoides appears to be the descendant of the wild diploid species that gave rise to the G genome of the tetraploids T. araraticum (T. timopheevi subsp. armeniacum) and T. timopheevi subsp. timopheevi (AAGG genomic formula).

x

3

Hence, the wild T. turgidum subsp. dicoccoides (wild emmer) has the AuAuBB genomic formula (Zohary and Hopf 2000), T. urartu being the male parent (Dvorák and Zhang 1990, Dvorak et al 1993, Brandolini et al 2006) and the donor species close to the current Ae. speltoides being the female parent. Wild emmer is still present in oak-­forest and herbaceous plant formations in Southeast Asia (Feldman and Kislev 2007). It thrives in the Fertile Crescent, from Israel and Jordan to the Zagros Mountains in southwestern Iran, through the Tigris and Euphrates basin in southeastern Turkey and northern Iraq (Feldman and Kislev 2007).

The wild allotetraploid T. araraticum (T. timopheevi subsp. armeniacum) (AuAuGG) can be collected in the eastern part of Turkey and Iran. It is morphologically very close to T. dicoccoides (Özkan et al 2002). It has been assumed that T. araraticum is a more recent species than T. dicoccoides.

The D genome of bread wheat has been identified as being very close to the genome of the wild diploid species Ae. tauschii (syn. Ae. squarrosa) (Kimber and Feldman 1987).

The Fertile Crescent is considered to be the cradle of these wild species (Bozzini 1988, Salamini et al 2002) because of the distribution and diversity in this region of diploid cultivated T. monococcum (taken as a possible representation of the past T. boeoticum and T. urartu distribution), of Ae. speltoides, of wild tetraploid wheats (T. turgidum subsp. dicoccoides and T. timopheevi subsp. armeniacum) and of Ae. squarrosa (the donor of the D genome in hexaploid wheats). This area of distribution

Fig. 1.1. Schematic representation of the origin of the chromosomes of cultivated wheats. An ancestral species diverged into different diploid progenitors that crossed. The resulting interspecific hybrids spontaneously doubled their chromosome stocks by natural polyploidy and gave rise to new polyploid species. As a result, Triticum aestivum has three homeologous copies of each of the seven ancestral chromosomes. Please note that this simplified scheme does not take into account chromosome rearrangement that occurred during polyploid evolution. Each bar is a pair of chromosomes.

4

x

Durum Wheat: Chemistry and Technology, 2nd ed.

is possibly more restricted than in the past, particularly for the wild species, which have been pushed away from the cultivated areas. However, the present area of distribution of these species is fully in line with the hypothesis of the speciation of wheats.

Domestication and differentiation In diploid species, two wild species (T. monococcum subsp. aegilopoides and T. urartu) and a cultivated species (T. monococcum subsp. monococcum) are represented. The situation is similar among tetraploid wheats: T. turgidum subsp. dicoccoides (AABB) and T. timopheevi subsp. armeniacum (AAGG) are wild species, while T. turgidum subsp. durum and T. timopheevi subsp. timopheevi are cultivated. The timopheevi wheat is endemic to western Georgia in Transcaucasia (Mori et al 2009).

Only cultivated hexaploid species have been described; they are T. aestivum (AABBDD) and T. zhukovskyi (AbAbAuAuGG) cultivated in Western Georgia. Furthermore, the A and B genomes of bread and durum wheat are almost identical and indicate a very recent common origin. Recent history and particularly domestication is thus very important to understanding the current organization of the diversity of modern wheats.

Crop Cereal Emergence During the Neolithic Period The earliest evidence of humans consuming wild wheats and wild barley as a food source is very ancient (about 19,000 years ago) and has been documented along the southwestern shore of the Sea of Galilee (Feldman and Kislev 2007). Extensive endemic stands of these wild cereals permitted the collection of a large amount of grain, especially wheat, with minimal labor and time (Ladizinsky 1975). At these times, wild cereals were gathered, and plants were not cultivated or domesticated.

The first basic steps toward the development of domesticated cereal crops were made when humans not only gathered the wild plant seeds but started sowing some of these gathered seeds.

Early humans then adopted the first measures for soil clearing and tilling and discovered the best time for seeding. The next step for domestication of the wild wheat was the selection of plants showing 1) a lower degree of spike brittleness and 2) larger kernel sizes, thus providing a higher amount of starch and proteins. Most likely, wheat straw was also collected from the start, representing a feed for animals, fuel for cooking fires, or a component, with clay, of materials to build dwellings.

Domestication is the sum of the selection processes, conscious or unconscious, that modify the wild plant into a crop that meets human needs for food production (and other needs as well), accompanied by adaptation to easier cropping. The spikes of wild plants have a brittle rachis with disarticulated spikelets.

They have hulled seeds, indeterminate tillering and asynchronous flowering, small grains with a high protein vs. starch ratio, cold-­season growth habit, and many other characteristics that are the opposite of those of current modern wheats. Modern wheat (durum and bread wheat) plants have a reduced number of tillers, synchronous flowering, and a solid rachis permitting the harvest of the whole spike. Grains are free-­threshed from the glumes; kernels are larger than in the wild forms and are less rich in protein (have a higher carbon-­to-­nitrogen ratio). These

changes are often described as the domestication syndrome that converts a wild species to a crop dependent on humans for its propagation and survival.

Archaeological and genetic evidence is focused on a period about 10,000 years ago in the Fertile Crescent (Zohary and Hopf 2000, Salamini et al 2002; for a review, see Kilian et al 2009). The protracted model, which considers that wheat domestication was a slow process and lasted at least one millennium (Tanno and Willcox 2006) is now accepted for cereals; the situation is still controversial for pulses (Abbo et al 2008). Domestication did not start abruptly; it was preceded by cultivation of wild populations.

Around 10,300–9,500 b.c.e., there are indications of cultivation of wild emmer in the southern Levantine Corridor in Israel, Jordan, and Syria and in the northern part of the Fertile Crescent (Syria, Turkey, Iraq, and Iran) (Feldman and Kislev 2007).

Domestication of wheats and barley, as well as of other wild grain legumes (lentils, peas, and chick peas) resulted from the settling of local people, evolving from the “hunter-­gatherers” to the “farmers” phase. Southeastern Turkey, in the Karakadag Mountain range, is a place where the three essential factors in species domestication (genetic, archaeological, and cultural practices) converge. This locality may have become the crucible for initial domestication for several species, including cereals (barley, rye, wheat) and legumes (pulses, Cicer spp.) (Salamini et al 2002, Özkan et al 2011).

Successful cultivation improved the well-­being of humans and their settlement. Consequently, higher levels of social life evolved, including specialized activities and the development of the first artisans and tradesmen. Starch and plant proteins in grain form represented a source of food easy to store without continuous care and reduced, or even eliminated, the need to search for the forage required by the feeding of domesticated animals. Cereals provided an available food, easy to prepare and of good nutritional value.

The Diploid Einkorn The wild diploid T. monococcum subsp. aegilopides (T. boeoticum) has been domesticated into T. monococcum subsp. monococcum, known as einkorn or “petit engrain” (reviewed by Kilian et al 2009). This cropped wheat has a relatively tough rachis, and its seeds (generally only one per spikelet) are almost twice the size of those of the wild T. boeoticum. The kernels of T. monococcum are tightly covered by the glumes, and the yield is rather poor (half a ton of covered grains per hectare). In some areas, it was cultivated also for straw and used for making handicrafts. This wheat is still cultivated in some remote mountainous areas of Italy, the Balkans, Turkey, and Transcaucasia, often mixed with other cereals; however, it could soon disappear from cultivation.

Its conservation, in germplasm banks as well as in fields, is very important for the improvement of all wheats since it carries an A genome close to that of polyploid cultivated wheats. In general, diploid wheats carry several genes resistant to many plant diseases (particularly rusts); show a profuse tillering characteristic, narrow leaves, and flexible stems; and are resistant to lodging.

The Karakadag area, in southeastern Turkey, has been proposed as the location of the initial domestication steps of T. monococcum subsp. aegilopoides (Heun et al 1997). Genetic data

Origin and Distribution of Genetic Diversity suggest a monophyletic origin for T. monococcum (Kilian et al 2007), but it is not yet clear whether domestication took place only once in a single locality from a unique founding population.

Wild Tetraploids and Domestication Most certainly, wild tetraploid wheats were already largely distributed in the Near East when humans started harvesting them from wild populations. Their general size, and particularly a larger head and kernels than those of T. monococcum subsp. aegilopoides, made them much more worthwhile for harvesting (and, later on, for cropping) than diploid wheats. Moreover, the spikelet was structured to differentiate two or three flowers, each spike consisting of 15–20 spikelets, which could in theory provide 30–60 kernels. Therefore, from early on, the potential of tetraploid wheats (as well as barley, rye, and oats) as a source of basic food was quite appealing to humans. From a morphological point of view, the wild T. dicoccoides types are not clearly distinguishable from T. timopheevi subsp. armeniacum populations, now scattered in the Zagros Mountains on the border between Iraq, Iran, and Armenia and in the Kurdistan areas of Turkey, Syria, and Iran. These are probably remnants of more largely distributed populations of wild tetraploid wheats.

The wild tetraploid T. turgidum subsp. dicoccoides was first domesticated into a form having a tough rachis but hulled grain (i.e., T. turgidum subsp. dicoccum or cultivated emmer), while T. timopheevi subsp. armeniacum similarly gave rise to the cultivated T. timopheevi subsp. timopheevi. Genetic data also suggest

x

5

a monophyletic origin of T. turgidum subsp. dicoccum (Özkan et al 2002, Luo et al 2007), strongly arguing in favor of a core area for domestication of cereals in the Karakadag Mountains (Salamini et al 2002). Gene flows between wild and domesticated forms modified their diversity patterns when they were dispersing across southwestern Asia (Luo et al 2007). Archaeological evidence also suggests that cultivation of wild emmer in the Levantine Corridor occurred several hundred years before domesticated emmer appeared, and different wild gene pools may have intercrossed to finally create the domesticated gene pool (Tanno and Willcox 2006, Feldman and Kislev 2007).

Dissemination of Cultivated Emmer and Landrace Differentiation It has been hypothesized that, after domestication, early farmers established a primitive agricultural system based on wheat, barley, legumes, and animals within the Fertile Crescent and then outward to the east and west (Nesbitt and Samuel 1996, Luo et al 2007). The rate of spread was linked to the migrations of farmers themselves and has been estimated to be about 1.5 km per year (Ammerman and Cavalli-­Sforza 1984). The local availability of these special types of grasses growing in large swards with spikes enclosing relatively large kernels started the expansion and growth of the western Asian civilization.

Emmer wheat (T. turgidum subsp. dicoccum) followed different dissemination pathways (Fig. 1.2) (Nesbitt and Samuel 1996, Luo et al 2007, Zaharieva et al 2010). During expansion, local

Fig. 1.2. Map of the diffusion pathways of Triticum turgidum subsp. dicoccum, from archaeological evidence. The star shows the center of origin of the species and the arrows the presumed ways of diffusion. The bold dotted lines relay a site of the same date, and the distance between them suggests the speed of the diffusion to the West and the North. (Reprinted from Zaharieva et al 2010, with permission from Springer Science+Business Media)

6

x

Durum Wheat: Chemistry and Technology, 2nd ed.

populations of farmers cultivated their own seeds across diversified environments and used the grains for different purposes (e.g., to prepare soup, bread, bulgur, semolina, and pancakes).

Human and natural selection, combined with reproductive isolation and assisted by random mutations, modified the genetic composition of their wheat populations, leading to the emergence of differentiated and specialized landraces among and within regions. Domesticated emmer differentiated into two principal groups of landraces. In the northern group, ecotypes from eastern Turkey, Transcaucasia, and Iran remained relatively close to the origin of their domestication. This group of emmer landraces were carried westward into Western Turkey, Greece, the northern Balkans (Serbia, Bosnia, and Croatia), and the Yaroslav region of northern Russia (Luo et al 2007).

A southern group of domesticated emmer ecotypes is made up of landraces from Ethiopia, Oman, southern India, and the Levant. Ethiopian emmer might be a remnant of Egyptian emmer. Archaeological remains demonstrated its presence in Transcaucasia, Central Asia, and the Nile Valley. In Egypt, emmer wheat is recorded about 8000–7000 b.c.e. Agriculture resulted in the development of the Old Egyptian and, later on, the Mediterranean and European civilizations. In Europe, emmer followed both a Mediterranean pathway (Greece, Italy, Spain) and a Black Sea-­Danubian path (Nesbitt and Samuel 1996). This first domesticated tetraploid wheat finally arrived in the Netherlands, Germany, and Poland about 4,500 b.c.e. and in Britain in 3500 b.c.e.

Rise of the Modern T. turgidum subsp. durum and Others with Free-­Threshing Subspecies New forms of tetraploid wheats continuously differentiated from emmer under the selection pressure imposed by local environments and cultural practices across a much larger distribution area than that of its wild ancestor T. dicoccoides. Some of these remarkable forms are still recognized today. Among these new forms is the modern T. turgidum subsp. durum (hereafter called T. durum or durum wheat for simplicity).

An important change appeared about 9,000–7,500 years ago with the first free-­threshing grains (Zohary and Hopf 2000).

In T. turgidum subsp. dicoccum, as in the wild dicoccoides, the kernel (caryopsis) is closely and tightly covered by the glumes, hence providing protection for the seed. Spontaneous mutations allowed the kernels to be more easily separated from the glumes (free-­threshing), leaving the seed “naked” and therefore easier to use, as the seeds of durum and bread wheats appear nowadays. Modern durum first appeared in the archaeological record in Egypt during the Greco-­Roman times (reviewed in Nesbitt and Samuel 1996). The genetic relationships are consistent with archaeology and suggest that durum evolved in the eastern Mediterranean. The modern form of T. durum gradually replaced the emmer, T. dicoccum, and established as a major crop during the Hellenistic period (2300 b.c.e.). Since this period, emmer wheat has been grown only on a limited scale in Ethiopia, India, Iran, Italy, Spain, Eastern Turkey, and the Balkans.

Other naked forms derived from emmer (with AB genomes) and differentiated only by some morphological traits include the other T. turgidum subspecies: subsp. paleocolchicum (or T. geor-

gicum); subsp. carthlicum (or T. persicum); subsp. turanicum (or T. orientale); subsp. polonicum (or T. ispahanicum); and subsp. turgidum (or pollard wheat). T. turgidum has soft grain, while T. polonicum (L.) Thell has a long and thin glume and a comparatively longer kernel than T. durum, determined by a special allele at the P locus located on chromosome 7AL (Wang et al 2002). All these naked turgidum types (with the possible exception of T. carthlicum) form a taxonomic subgroup clearly separated from T. dicoccum (Thuillet et al 2005). Later, the more advanced types (characterized by naked kernels and much wider adaptation) belonging to T. turgidum and T. durum spread to all of Europe, the Middle East, and North Africa. During the Roman Empire, most of the wheat carried to Rome from the colonies belonged to the dicoccum-­turgidum-­durum group. The basic difference between the turgidum and durum types is the kernel structure (starchy in turgidum and vitreous in durum, somewhat parallel to the dent and flint types in maize) and in the better adaptation of the durum types to warm, semiarid conditions and of turgidum types to a more continental, cold, and humid climate.

Both types were used for making bread (in the many different types, both unleavened and leavened, available in the past and even nowadays in all Mediterranean, Near and Middle East, and Ethiopian areas) or for a number of other uses, some of which are still common today (bulgur, couscous, chapati, injera, etc.) besides the more recent pasta.

Thousands of years of cultivation and continuous natural and human selection have resulted in a tremendous morphological and adaptive variability in the tetraploid wheats derived from wild emmer. Among all the cultivated tetraploid wheats, T. durum types are by far the most important ones, even though they are grown in only 10% of all the wheat-­cultivated area, the remaining 90% being represented by the hexaploid bread wheat, T. aestivum (Darlymple 1978, Hanson et al 1982).

Durum Wheat Evolution Under Modern Genetic Improvement The long breeding history of durum wheat has allowed the cultivated germplasm to differentiate morphological characteristics and adaptability to distinct agricultural environments. A taxonomic classification system was proposed in the middle of the twentieth century (De Cillis 1964) identifying different types (sections, sometimes called “proles” in an ancient terminology).

• European types are adapted to regions with continental climates that are relatively cold, like those of Eastern Europe, Anatolia, and some of the Middle East countries.

They are characterized by the more-­or-­less strong need for vernalization and high biomass, with stem heights of about 150–170 cm. German immigrants to the New World brought with them the seeds of Eastern European durum landraces that founded the North American cultivars.

• Mediterranean/African types diffused into the Mediterranean Basin and North Africa. They have medium water needs during the first phases of the vegetative cycle, high growth rates (height, 150 cm), large, long leaves, and large stems.

• Syro-­Palestinian types (in the semiarid Near East) are characterized by low water requirements in all phases,

Origin and Distribution of Genetic Diversity earliness, and narrow plant structure (lower height, about 120 cm, and thinner stems).

• Abyssinic types are morphologically more diversified, with thin stems and colored and smaller kernels.

Over the last century, crop improvement programs have produced elite productive varieties, specific for areas where agriculture is technologically advanced. In durum wheat, as in other self-­fertilized cereals, the initial step in breeding advanced genotypes has first involved a phase of selections among locally cultivated populations (landraces), followed by crossbreeding among these promising lines, followed by selection for an extensive range of characters (e.g., yield, quality, and disease resistance) within the genetically heterogeneous subsequent generations.

Historically, the most important breeding activities with durum, which have led to the best outcomes, have been performed in 1. Italy since the very beginning of the twentieth century (Bozzini et al 1998), 2. North America (North Dakota and Canada), 3. South America (Argentina), as well as 4. two international research centers: the International Centre for Wheat and Maize Improvement (CIMMYT), located in Mexico (Abdalla et al 1993, Pfeiffer et al 2000), and the International Centre for Agricultural Research in the Dry Areas (ICARDA), located in Aleppo, Syria.

These two centers have worked together in an important program of improvement of durum wheat, specifically aimed at the dry areas, particularly of the Mediterranean regions (Nachit et al 1998).

The initial breeding activities for the improvement of durum wheat, conducted during the first decades of the twentieth century, identified a relatively small number of “improved selections” out of the huge variability present in local populations of the Mediterranean Basin. In particular, Italian improvements explored the variability of national and North-­African populations (Libya and Algeria) and those of the Near East. The first crossbreds among selected materials date back to this period. Many of these materials were characterized by the good quality of the grain. Several varieties were developed in this period. Among these is the cultivar (cv.) Senatore Cappelli (of North African origin, of very good quality, and suitable for Mediterranean environments but characterized by high stems, late maturity, and susceptibility to several diseases). Another is cv. Capeiti 8 (coming from the cross between Senatore Cappelli and a selection of Eiti, a Near East line characterized by earliness, shorter stems, and adaptability to semidry conditions).

As a result of changes to agronomic practices and particularly the increased use of nitrogenous fertilizers, the genotypes selected from local populations have demonstrated problems of excessive lateness and vegetative biomass. This has led to a higher susceptibility to rusts and to late ripening. Moreover, the plants have been exposed to damage caused by “terminal heat stress,” typical of the Mediterranean environment. Only the materials similar to Capeiti 8 (qualitatively worse) showed some adaptation, and thus, toward the second half of the twentieth century, their cultivation increased across Italy with the decreasing use of S. Cappelli. However, S. Cappelli was largely used as a gene donor for the durum improvement programs in different areas

x

7

of the world, in particular, in the areas of the Mediterranean Basin and by CIMMYT. Toward the second half of the twentieth century, durum wheat breeders aimed at reducing plant size and improving crop production through earliness and an intensified tillering ability, associated with spike fertility. Besides crossbreeding among improved varieties, there was an attempt to utilize mutagenesis and interspecific crossbreeding with different kinds of Triticum—­ above all, with bread wheat. One of the most important events was the transfer into durum wheat of the semidwarf growth habit, principally coming from the bread wheat variety Norin 10 (particularly gene Rht1 located in chromosome 4A).

The improvements made in this period, principally by CIMMYT and in Italy and other Mediterranean countries, created an elite genetic pool adapted to temperate-­hot environments. This pool was characterized by smaller plant size, with high and medium earliness, insensitivity to photoperiod, and high production ability under quite favorable conditions.

The greatest improvement in size reduction and earliness was reached through some important CIMMYT (e.g., Cocorit 71 and Mexicali 75) and Italian (e.g., Creso) cultivars, which spread in different areas of durum wheat cultivation (principally in the Mediterranean Basin). The yield potential of the durum wheat in this area soon reached that of bread wheat.

On the contrary, materials developed in North America have been and are selected to maintain medium height and sensitivity to photoperiod. These are characteristics that foster better adaptation to spring cultivation conditions in the North American plains.

In the last few years, the pasta industry has driven the breeder’s attention toward improvement of the technological quality of durum. A series of grain characteristics (protein content, yellow pigmentation, and gluten characteristics) have become the subject of selection and genetic improvement. Reaching a high standard of quality has become one of the primary aims of the CIMMYT improvement programs, both in North America and in Europe (Clarke et al 1998, Peña et al 2000). The improvement in yield potential has led to new progress being made by CIMMYT through selection for productivity and wide adaptability (e.g., cv. Yavaros 79) and also through the use of breeding strategies such as “development of the ideotype” with cv. Altar 84 (e.g., through introgression of the “upright leaf” character and improvement of the photosynthetic efficiency).

For areas subjected to environmental stresses, where short-­ stemmed varieties are not so well adapted, ICARDA has achieved good results after characterizing and using local Near East populations and wild species (ICARDA 1995–1997).

Pecetti and Annicchiarico (1998) analyzed the agronomic value and the morphophysiological characteristics of four groups of varieties of Italian durum wheats belonging to the early stages of the Italian breeding activities. In the first group were the local native landraces and, in the second, the selected materials, including the exotic ones brought into Italy during the first genetic-­improvement stages. In the third group were the selected genotypes obtained through breeding processes or mutagenesis from materials of the second group, and finally, in the fourth group, were modern and recent materials obtained through breeding with materials coming from CIMMYT.

8

x

Durum Wheat: Chemistry and Technology, 2nd ed.

This study, conducted in a moderately favorable environment, highlighted the constant increase in genetic potential of the groups of materials from group 1 to group 4, due both to the improvement of the number of grains per spike and the medium weight of the grains and to the increase of tillering and fertile stems. A decrease in plant height, the extension of the period of grain filling, and the increase of the early tillering have also been observed. However, the genetic gain from group 3 to group 4 was the smallest. This is probably due to the strong genetic and phenotypical uniformity that characterizes the modern genetic pool.

Compared to the wide genetic variability that characterizes local populations, analysis of the pedigrees of the plants with improved germplasm has pointed to a relatively low number of “founding” or “ancestral” accessions among the landraces or their first selections. Only a few varieties have played a relevant role in the creation of the genetic basis of modern genetic pools (for improved accessions depending on the program CIMMYT-­ICARDA: Autrique et al 1996; for the Italian genetic pool: Bozzini et al 1998; for the North American genetic pool: Joppa and Williams 1988). However, Pfeiffer et al (2000) showed that durum wheat varieties released recently by CIMMYT still showed genetic advancement. This positive outcome has been achieved through the development of different ideotypes. The most recent varieties have not been characterized by increases in the “harvest index,” but rather by a higher growth rate of the biomass of all the components of the upper part of the plant and a relative lateness in flowering and ripening (the highest earliness had been reached with the Mexicali variety in 1975), the latter associated with an increase in the number of grains per spike and of spikes per square meter.

To maintain the rates of increase in yield potential of future durum cultivars, breeders may be forced to resort to more complex approaches to introducing genetic variability into their experimental material. For example, they may employ methods such as 1. the targeted use of the alien germplasm through substitutions and chromosomal translocations obtained through meiosis control and recombination, 2. the creation of special genetic stocks characterized by extreme expression of single characters that control the potential yield (major genes), originating from alien donors, and 3. the use of synthetic hexaploids with AAB and ABB genomes combined with the study of new methodologies to obtain heterotic combinations.

DNA Marker Analysis of Modern Wheats Molecular markers are helpful for deciphering the genetic structure of the modern varieties resulting from twentieth century breeding activities. A representative collection of 134 varieties of durum wheat cultivars were genotyped using a set of 70 microsatellite markers; these varieties could be grouped into six to eight main distinct subpopulations (Maccaferri et al 2005). Geographical origin accounted only for 20% of the variation. The most striking feature was the similarity between North American and Mediterranean cultivars. CIMMYT-­ ICARDA

and Italian cultivars appeared to be very closely related. French varieties overlapped the North American and the Italian and CIMMYT-­ICARDA gene pools. Tracing back allele genealogy, the diversity of recent cultivars appears to come from a small number of elite “founding” cultivars (Maccaferri et al 2003).

A complementary study on 189 accessions from ICARDA, CIMMYT, Spain, Morocco, Tunisia, and Arizona/California irrigation areas was conducted by Natoli (2008). The within-­ group variation again accounted for the largest portion of the total variation (82–95%). This might be expected since materials of different origins are shared between breeders, as confirmed by pedigree analysis. However, some grouping within this germplasm collection appeared (Fig. 1.3). An “S1” group corresponded to a clearly differentiated group of accessions coming from ICARDA that were based on some Syrian varieties and consisted of a few accessions developed for the very dry part of Eastern Syria. The identified Syrian founding varieties were a recent cultivar Omrabi obtained by ICARDA, two “Syriacum durum types” Haurani and Eiti, and Capeiti8, obtained in Italy from Eiti crossed with Cappelli, an Italian cultivar.

Groups “S2,” “S3,” and “S4” showed a low value of differentiation. The group “S2” contained a large number of accessions, consisting of some experimental lines recently obtained by ICARDA for temperate areas. This material is characterized by high production potential; its principal representative is the cultivar Cham1. Some important Italian early varieties like Cappelli and Creso, both used by ICARDA and CIMMYT in their breeding programs, belong to this group.

The “S3” group was principally represented by Italian cultivars derived from early varieties like Valnova (Italian) and Mexicali 75 (CIMMYT). A series of cultivars and lines, coming from ICARDA, Italy, Spain, Morocco, and Tunisia, belonged to group “S4.” This breeding material, which originated from CIMMYT, is characterized by high environmental adaptability and production stability. It is centered on a Jori/Anhinga// Flamingo group (“Bittern” group, released toward the end of the 1970s).

The last group, “S5,” included some accessions coming from ICARDA, CIMMYT, Italy, Spain, and Morocco, derived from the founder, CIMMYT’s Gallareta (same as Altar84), and characterized by high yield potential. It can be considered the most recently CIMMYT-­developed germplasm.

Bread Wheat and Hexaploid Forms Visually distinguishing between tetraploid and hexaploid naked grains is not easy. Dating the first occurrence of hexaploid wheat in archaeological remains is still a challenging issue. T. aestivum appears to have originated near the south or west of the Caspian Sea (Dvorak et al 1998) and resulted from a reduced number of spontaneous hybridizations (Talbert et al 1998, Blake et al 1999) between a tetraploid wheat of genomic formula AABB (probably emmer) and the goat grass Ae. tauschii (syn. Ae. squarrosa (L)). This brought the D genome into the bread wheat genome after spontaneous chromosome doubling (2n = 6x = Au AuBBDD) (Kihara 1944). The cross likely occurred about 8,000 years ago in this area where Ae. tauschii still thrives today.

Origin and Distribution of Genetic Diversity

Fig. 1.3. Dendrogram of the genetical relationships among 189 varieties of durum wheat. A Modified Rogers’ distance was computed from 191 simple sequence repeat loci, and the UPGMA method was used to build the tree. The scale of values, in percent, indicates the similarity between the accessions. (Reprinted, with permission, from Natoli 2008)

x

9

10

x

Durum Wheat: Chemistry and Technology, 2nd ed.

Within T. aestivum, several subspecies have been described (subspp. spelta, vavilovii, macha, compactum, and sphaerococcum), representing the most common types of bread wheat now cultivated (see https://www.ksu.edu/wgrc/Taxonomy/taxintro.html.)

Another hexaploid species exists: T. zhukovskyi (genome AbAbAuAuGG), in which the A genome is represented twice. It could have been derived from a cross between T. timopheevi and T. monococcum, followed by a doubling of the chromosome number.

Triticale During the twentieth century, new cereal species were created from crosses of tetraploid or hexaploid wheats with rye (Secale cereale L., genome R) to produce triticale (xTriticosecale Witt.). Great interest has arisen, particularly in the hexaploid triticale (genome AABBRR), not only from a scientific but also from an economic point of view, since it is relatively cytologically stable (unlike some octoploid triticales). This new hexaploid species expresses characteristics derived from both parental species (normally durum and rye), such as disease resistance and acid soil tolerance, earliness of heading, an extended period between flowering and ripening, and longer kernels. Durum wheat’s larger kernel size is dominant; the seeds of triticale are then normally larger and longer but not always completely filled.

This is also attributable to slower endosperm cell division and the consequent delay in kernel filling, resulting in kernel shriveling and possible yield depression. However, after many years of selection, higher yields are now obtained. Its acreage was 4,132,972 ha in 2009 (http://www.faostat.org).

Sources of genetic diversity for breeding Breeding requires genetic diversity. A reduction of the genetic differences in cultivated germplasm could lead to • genetic vulnerability to pests and pathogens due to the emergence of new mutant or recombinant pathotypes, • reduction of the germplasm plasticity for adaptation to cultural and/or climatic variations, and/or • reduction of the progress obtained through the selections of intercrosses of elite cultivars.

Role of Wild and Primitive Wheats Domestication influences the level of the genetic diversity of the new crop. As a small group of populations and individuals carrying the desired traits for human needs were selected from a vast wild population, the entire diversity available in the wild ancestor was not incorporated (causing a genetic bottleneck). It is assumed that no or limited genetic bottleneck occurred for T. monococcum subsp. monococcum when the Karakadag subsp. aegilopoides (T. boeoticum) population is taken as a reference (Kilian et al 2007). However, it is likely that AABB tetraploid wheats lost around 50% of the gene diversity during the transition from T. turgidum subsp. dicoccoides to T. turgidum subsp. dicoccum (Thuillet et al 2005, Haudry et al 2007). The same trend can be observed between T. timopheevi subsp. armeniacum and T. timopheevi subsp. timopheevi (Mori et al 2009).

On the other hand, domestication and cultivation across large and diverse areas of the Fertile Crescent may have contributed to a higher-­t han-­expected level of variability for some characters among and within wheat landraces across thousands of generations. This variability has increased further in tetraploid and hexaploid wheats because of their ploidy level and by the survival of spontaneous mutations, if these mutations favor human utilization of these cereals. For example, new alleles were found in the cultivated pool at a resistance gene to powdery mildew (Blumeria graminis f. sp. tritici; also called Erysiphe graminis) (Yahiaoui et al 2006). Mutant alleles associated with new adaptations to localized environments were selected. Such alleles with major effect have been cloned in barley and rice but not many in wheat because of its genome size and complexity (reviewed in Kilian et al 2009). Because of the polyploid status of wheats, homeologous copies govern the expression of traits and can have dosage effects.

Using colinearity between grasses and particularly with barley, rapid progress is expected in the identification of genes responsible for the major transitions. In maize, 3–4% of the genes are suspected to have evolved under selection during the wild-­ to-­crop transition (Wright et al 2005); the trend in wheat might be of a similar order.

In the transition from the hulled to the current naked wheats, genetic diversity also experienced a strong reduction, and globally all naked tetraploid wheats share a close genetic relationship (Thuillet et al 2005, Haudry et al 2007). Furthermore, T. durum experienced an even stronger reduction of diversity in the micro­satellite locus near the Tg2 locus on chromosome 2B, responsible for the tenacious glume phenotype (Simonetti et al 1999). Thuillet et al (2005) suggested that a strong selective pressure accompanied the transition from T. turgidum subsp. dicoccum or T. turgidum subsp. parvicoccum (now extinct) to T. turgidum subsp. durum.

The relatively low level of diversity of T. durum may be compensated for by the use of the gene diversity available in the other taxa of AABB genomes. Large collections of these genetic resources are available in various gene banks, either national (IPK Gatersleben, Germany; Bari, Italy) or international (ICARDA and CIMMYT).

An interesting variability in morphophysiological and quality characters and resistance to biotic and abiotic stresses has been found (Table 1.1). However, for quality traits such as semolina yield, dough strength, or yellow color, wild and primitive germplasm does not provide much genetic variability. In the introgressed lines, T. turgidum subsp. dicoccoides alleles may also have detrimental impacts on technological properties (weak in dough strength, poor yellow color), which may be corrected by backcrossing several times onto T. turgidum subsp. durum (Sissons and Hare 2002). Since the durum wheat genome is similar to those of these wild species, the transfer of desirable traits into the elite germplasm is realized by simple crosses and interchromosomal recombination. The use of this particular germplasm has led, at least partially, to the introgression of some undesirable characteristics. Some specific activities are needed, like pre-­breeding, which involves a preliminary accession assessment, crossbreeding, and the formation of a genetic stock with specific and useful traits. Such intro-

Origin and Distribution of Genetic Diversity gression programs are conducted by ICARDA and CIMMYT (Valkoun 2001).

Genetic Variability in Traditional Durum Wheat Landraces During the last century, the impact of breeding and of modern agronomic practices led to a corresponding decrease in the genetic diversity at the intraspecific level. This has been clearly demonstrated with durum wheat (Thuillet et al 2005). However, in less developed areas, local durum populations (landraces) are still frequently cultivated, since they are still better adapted to the local conditions and farmers’ needs than are commercial varieties (ICARDA 1999, 2000). These landraces retain alleles that have either disappeared or have never been incorporated into elite germplasm and therefore are a source of potentially useful alleles for modern breeding. A typical example of the important role of landraces in genetic improvement is related to the breeding program for bread wheat led by CIMMYT, where a great number of gene donors are represented by landraces or by genotypes of unknown origin.

The genetic diversity of landraces may be considered under two principal aspects: the difference between regions and popu­ lations (space heterogeneity) and the evolution of difference within regions and populations (time heterogeneity). The genetic structure of landraces has been studied in many cultivated species first using isoenzymatic and then molecular markers (Doebley et al 1985, Demissie et al 1998). The great difference among populations found using either isoenzymatic or molecular markers has been related both to the multiple environmental variations that affect these populations and to the relatively limited selective pressure to which they have been submitted.

For many cultivated species (above all, cereals), it is possible to trace, almost precisely, the original materials at the basis of the development of the elite germplasm through a retrospective analysis of pedigree information (Autrique et al 1996, Kisha et al 1998). Nevertheless, available information concerning both

x

the genetic structure of this original germplasm and the variability of its morphophysiological characteristics is still scarce.

Landraces are still considered of interest for the presence of single genes that are favorable to adaptation and productivity, but they are also of interest for gene complexes, that is, complex epistatic interactions that have been fixed after a high number of selective generations. However, besides the presence of single genes, landraces could also have other desirable characteristics under polygenic control (e.g., height reduction, higher seed weight, higher number of grains per spike, better nutrient absorption, etc.), as documented in bread wheats derived from crosses with cv. Norin 10. It has been proposed that the complex combination of these characters was established as the result of adaptation and selection in irrigation farming systems several centuries ago in bread wheat landraces cultivated in China and Japan (Kihara et al 1949, Frankel et al 1995).

Today, locally grown cultivars do not play a contributing role in genetic improvement programs, as they have been replaced both by improved populations and by an increasing use of genetic resources coming also from wild types. Landraces, from selected areas, are mainly considered gene donors, both of specific characters whose genetic control is rather simple (for example, resistance to biotic stress) and of complex characters linked to the adaptation and inherited as “supergenes or association gene blocks.”

Genetic Variability in the Elite Germplasm Despite the potential value of landraces and wild wheats, adapted elite germplasm still represents the most utilized genetic resource for crop improvement. This is justified by clear evidence of continuous genetic progress in the production potential of the new varieties (Pfeiffer et al 2000). Moreover, to enlarge the genetic basis of the elite germplasm (e.g., to improve the resistance to biotic and abiotic stresses), it is preferable, when possible, to use elite germplasm developed in varying areas (“exotic” material) rather than using only local landraces or even more exotic

TABLE 1.1 Useful Traits for Durum Improvement Documented in Triticum turgidum Relatives Subspecies

Trait

Reference

T. turgidum subsp. carthlicum T. turgidum subsp. dicoccoides

Powdery mildew resistance Powdery mildew resistance

T. turgidum subspp. carthlicum and dicoccum T. turgidum subspp. dicoccoides, dicoccum, carthlicum, and turanicum T. timopheevii subsp. armeniacum and T. turgidum subsp. carthlicum T. turgidum subsp. turanicum (Khorasan wheat) T. turgidum subsp. polonicum T. turgidum subsp. dicoccoides T. dicoccoides

Fusarium head blight resistance Resistance to tan spot and Stagonospora nodorum blotch, (Pyrenophora tritici-repentis, Stagonospora nodorum) Hessian fly resistance

Zhu et al (2005) Ji et al (2008) Blanco et al (2008) Somers et al (2006) Chu et al (2008a,b) Singh et al (2006) Nsarellah et al (2003)

1,000-kernel weight Salinity tolerance Glaucousness (ability to stay green) Grain protein, zinc, and iron content High-temperature adult-plant stripe rust resistance gene Low molecular weight glutenin High molecular weight glutenin

Sissons and Hare (2002) Munns and James (2003) Simmonds et al (2008) Uauy et al (2006) Uauy et al (2005) Li et al (2008) Xu et al (2009)

T. turgidum subspp. dicoccoides and dicoccum T. turgidum subsp. polonicum

11

12

x

Durum Wheat: Chemistry and Technology, 2nd ed.

germplasm. In some cases, though, a decrease in the rate of genetic improvement has been noted. However, increased productivity is still necessary, because of the rapid global increase in the demand for cereals (Hoisington et al 1999, Miflin 2000).

Achieving this aim will probably require some innovations in breeding techniques. In barley, tetraploid wheat, and rice, for example, hybrid varieties based on heterotic groups could represent a valid alternative to pure lines. In durum wheat, such heterosis is much lower than in maize, with the best yield of hybrids being around 15% more than that of the best pure lines (Widner and Lebsock 1973). However, this heterosis was considered sufficient to justify the development of commercial hybrid varieties.

The main difficulty is hybrid seed production, which relies on an efficient emasculation (a chemical method was used for commercial varieties), a good overlap of male and female flowering, and a suitable floral structure that allows outcross pollination in field conditions for a usually selfing species. The optimization of the heterosis levels may also require the creation of heterotic groups selected for their combining ability (Hoisington et al 1999). A possible and interesting way could be to find these heterotic groups based on knowledge of the wider genetic structure of the T. turgidum gene pool. An ultralong-­term vision for durum wheat breeding in the future might involve reconstituting and domesticating a new T. durum utilizing accessions of wild T. turgidum subsp. dicoccoides, repeating in plant breeding plots what nature produced over millennia in the wild. Such a pool could be heterotic on the current durum lines and used in hybrid combinations.

References

Abbo, S. I., Zezak, I., Schwartz, E., Lev-­Yadun, S., and Gopher, A. 2008. Experimental harvesting of wild peas in Israel: Implications for the origins of Near East farming. J. Archaeol. Sci. 35:922-­929.

Abdalla, O. S., Peña, R. J., Autrique, J. E., and Nachit, M. M. 1993.

Durum wheat breeding and quality improvement at CIMMYT Mexico. Pages 133-­ 141 in: Durum Wheat Quality in the Mediterranean Region. (Options Méditerranéennes, Series A, Vol. 22.) N. Di Fonzo, F. Kaan, and M. Nachit, Eds. Int. Centre for Advanced Mediterranean Agronomic Studies (CIHEAM), Zaragoza, Spain.

Akhunov, E. D., Akhunova, A. R., Linkiewicz, A. M., Dubcovsky, J., Hummel, D., Lazo, G. R., Chao, S. M., Anderson, O. D., David, J., Qi, L. L., Echalier, B., Gill, B. S., Miftahudin, Gustafson, J. P., La Rota, M., Sorrells, M. E., Zhang, D. S., Nguyen, H. T., Kalavacharla, V., Hossain, K., Kianian, S. F., Peng, J. H., Lapitan, N. L. V., Wennerlind, E. J., Nduati, V., Anderson, J. A., Sidhu, D., Gill, K. S., McGuire, P. E., Qualset, C. O., and Dvorak, J. 2003.

Synteny perturbations between wheat homoeologous chromosomes caused by locus duplications and deletions correlate with recombination rates. Proc. Natl. Acad. Sci. U.S.A. 100:10836-­10841.

Ammerman, A. J., and Cavalli-­Sforza, L. L. 1984. The Neolithic Transition and the Genetics of Populations in Europe. Princeton University Press, Princeton, NJ. Autrique, E., Nachit, M. M., Monneveux, P., Tanksley, S. D., and Sorrells, M. E. 1996. Genetic diversity in durum wheat based on RFLPs, morphophysiological traits, and coefficient of parentage.

Crop Sci. 36:735-­742.

Badaeva, E. D., Dedkova, O. S., Gay, G., Pukhalskyi, V. A., Zelenin, A. V., Bernard, S., and Bernard, M. 2007. Chromosomal rearrangements in wheat: Their types and distribution. Genome 50:907-­926.

Blake, N. K., Lehfeldt, B. R., Lavin, M., and Talbert, L. E. 1999.

Phylogenetic reconstruction based on low copy DNA sequence data in an allopolyploid: The B genome of wheat. Genome 42:351-­360.

Blanco, A., Gadaleta, A., Cenci, A., Carluccio, A., Abdelbacki, A., and Simeone, R. 2008. Molecular mapping of the novel powdery mildew resistance gene Pm36 introgressed from Triticum turgidum var. dicoccoides in durum wheat. Theor. Appl. Genet. 117:135-­142.

Bozzini, A. 1988. Origin, distribution, and production of durum wheat in the world. Pages 1-­16 in: Durum Wheat: Chemistry and Technology, 1st ed. G. Fabriani and C. Lintas, Eds. Am. Assoc.

Cereal Chem., St. Paul, MN. Bozzini, A., Corazza, L., D’Egidio, M. G., Di Fonzo, N., La Fiandra, D., Pogna, N. E., and Poma, I. 1998. Durum wheat (Triticum turgidum spp. durum). Pages 181-­194 in: Italian Contribution to Plant Genetics and Breeding. G. T. Scarascia Mugnozza and M. A. Pagnotta, Eds. Tipolitografia Quatrini A.& F. snc, Viterbo, Italy.

Brandolini, A., Vaccino, P., Boggini, G., Ozkan, H., Kilian, B., and Salamini, F. 2006. Quantification of genetic relationships among A genomes of wheats. Genome 49:297-­305.

Chu, C.-­G., Xu, S. S., Faris, J. D., Nevo, E., and Friesen, T. L. 2008a.

Seedling resistance to tan spot and Stagonospora nodorum leaf blotch in wild emmer wheat (Triticum dicoccoides). Plant Dis. 92:1229-­1236.

Chu, C. G., Friesen, T. L., Faris, J. D., and Xu, S. S. 2008b. Evaluation of seedling resistance to tan spot and Stagonospora nodorum blotch in tetraploid wheat. Crop Sci. 48:1107-­1116.

Cifuentes, M., Grandont, L., Moore, G., Chèvre, A., and Jenczewski, E. 2010. Genetic regulation of meiosis in polyploid species: New insights into an old question. New Phytol. 186:29-­36.

Clarke, J. M., Marchylo, B. A., Kovacs, M. I. P., Noll, J. S., McCaig, T. N., and Howes, N. K. 1998. Breeding durum for pasta quality in Canada. Euphytica 100:163-­170.

Darlymple, E. D. G. 1978. Development and spread of high yielding varieties of wheat and rice in the less developed countries. Foreign Agric. Econ. Rep. 95. U.S. Dep. Agric., Washington, DC. De Cillis, U. 1964. The objectives required in breeding hard wheat and the means of attaining them. Genet. Agrar. 18:51-­70.

Demissie, A., Bjornstad, A., and Kleinhofs, A. 1998. Restriction fragment length polymorphisms in landrace barleys from Ethiopia in relation to geographic, altitude, and agro-­ecological factors. Crop Sci. 38:237-­243.

Doebley, J. F., Goodman, M. M., and Stuber, C. W. 1985. Isozyme variation in the races of maize from Mexico. Am. J. Bot. 72:629-­639.

Dubcovsky, J. M., Luo, J., and Dvorak, J. 1995. Differentiation between homoeologous chromosomes 1A of wheat and 1Am of Triticum monococcum and its recognition by the wheat Ph1 locus. Proc.

Natl. Acad. Sci. U.S.A. 92:6645-­6649.

Dvorák, J., and Zhang, H. B. 1990. Variation in repeated nucleotide sequences sheds light on the phylogeny of the wheat B and G genomes. Proc. Natl. Acad. Sci. U.S.A. 87:9640-­9644.

Dvorak, J., Diterlizzi, P., Zhang, H. B., and Resta, P. 1993. The evolution of polyploid wheats: Identification of the A genome donor species. Genome 36:21-­31.

Dvorak, J., Luo, M. C., Yang, Z. L., and Zhang, H. B. 1998. The structure of the Aegilops tauschii genepool and the evolution of hexaploid wheat. Theor. Appl. Genet. 97:657-­670.

Dvorak, J., Akhunov, E. D., Akhunov, A. R., Deal, K. R., and Luo, M. 2006. Molecular characterization of a diagnostic DNA marker for domesticated tetraploid wheat provides evidence for gene flow from wild tetraploid wheat to hexaploid wheat. Mol. Biol. Evol.

23:1386-­1396.

Feldman, M., and Kislev, M. E. 2007. Domestication of emmer wheat and evolution of free-­t hreshing tetraploid wheat. In: Special Issue:

Origin and Distribution of Genetic Diversity A Century of Wheat Research—­From Wild Emmer Discovery to Genome Analysis. Isr. J. Plant Sci. 55:207-­221.

Griffiths, S., Sharp, R., Foote, T. N., Bertin, I., Wanous, M., Reader, S., Colas, I., and Moore, G. 2006. Molecular characterization of Ph1 as a major chromosome pairing locus in polyploid wheat. Nature 439:749-­752.

Hanson, H., Borlaug, N. E., and Anderson, R. G. 1982. Wheat in the Third World. Westview Press, Boulder, CO. Haudry, A., Cenci, A., Ravel, C., Bataillon, T., Brunel, D., Poncet, C., Hochu, I., Poirier, S., Santoni, S., Glémin, S., and David, J. 2007.

Grinding up wheat: A massive loss of nucleotide diversity since domestication. Mol. Biol. Evol. 24:1506-­1517.

Heun, M., Schäfer-­Pregl, R., Klawan, D., Castagna, R., Accerbi, M., Borghi, B., and Salamini, F. 1997. Site of einkorn wheat domestication identified by DNA fingerprinting. Science 278:1312-­1314.

Hoisington, D., Khairallah, M., Reeves, T., Ribaut, J. M., Skovmand, B., Taba, S., and Warburton, M. 1999. Plant genetic resources: What can they contribute toward increased crop productivity?

Proc. Natl. Acad. Sci. U.S.A. 96:5937-­5943.

ICARDA. 1995, 1996, 1997, 1999, 2000. Annual reports. International Centre for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria. (http://www.icarda.cgiar.org/AnnReport.htm) Ji, X., Xie, C., Ni, Z., Yang, T., Nevo, E., Fahima, T., Liu, Z., and Sun, Q. 2008. Identification and genetic mapping of a powdery mildew resistance gene in wild emmer (Triticum dicoccoides) accession IW72 from Israel. Euphytica 159:385-­390.

Jiang, J., and Gill, B. S. 1994. Different species-­specific chromosome translocations in Triticum timopheevii and T. turgidum support the diphyletic origin of polyploid wheats. Chromos. Res. 2:59-­64.

Johnson, B. L., and Dhaliwal, H. S. 1976. Reproductive isolation of Triticum boeoticum and Triticum urartu and the origin of the tetraploid wheats. Am. J. Bot. 63:1088-­1094.

Joppa, L. R., and Williams, N. D. 1988. Genetics and breeding of durum wheat in the United States. Pages 47-­66 in: Durum Wheat: Chemistry and Technology, 1st ed. C. Fabriani and C. Lintas, Eds. Am. Assoc. Cereal Chem., St. Paul, MN. Kihara, H. 1944. Discovery of the DD-­a nalyser, one of the ancestors of Triticum vulgare. (In Japanese.) Agric. Hort. (Tokyo) 19:13-­14.

Kihara, H., and Lilienfeld, F. 1949. A new synthesized-­ wheat.

Hereditas 35(Suppl.):307-­319.

Kilian, B., Özkan, H., Walther, A., Kohl, J., Dagan, T., Salamini, F., and Martin, W. 2007. Molecular diversity at 18 loci in 321 wild and 92 domesticate lines reveal no reduction of nucleotide diversity during Triticum monococcum (einkorn) domestication: Implications for the origin of agriculture. Mol. Biol. Evol. 24:2657-­2668.

Kilian, B., Özkan, H., Pozzi, C., and Salamini, F. 2009. Domestication of the Triticeae in the Fertile Crescent. Pages 81-­119 in: Genetics and Genomics of the Triticeae. C. Feuillet and G. J. Muehlbauer, Eds. Springer, New York.

Kimber, G., and Feldman, M. 1987. Wild wheat. An introduction. (Special report.) College of Agriculture, University of Missouri, Columbia.

Kimber, G., and Sears, E. R. 1987. Evolution in the genus Triticum and the origin of cultivated wheat. Pages 154-­164 in: Wheat and Wheat Improvement, 2nd ed. E. G. Heyne, Ed. American Society of Agronomy, Madison, WI. Kisha, T. J., Diers, B. W., Hoyt, J. M., and Sneller, C. H. 1998. Genetic diversity among soybean plant introductions and North American germplasm. Crop Sci. 38:1669-­1680.

Ladizinsky, G. 1975. Collection of wild cereals in the upper Jordan Valley. Econ. Bot. 29:264-­267.

Levy, A. A., and Feldman, M. 2004. Genetic and epigenetic reprogramming of the wheat genome upon allopolyploidization. Biol. J. Linnean Soc. 82:607-­613.

x

13

Li, X., Wang, A., Xiao, Y., Yan, Y., He, Z., Appels, R., Ma, W., Hsam, S., and Zeller, F. 2008. Cloning and characterization of a novel low molecular weight glutenin subunit gene at the Glu-­A3 locus from wild emmer wheat (Triticum turgidum L. var. dicoccoides).

Euphytica 159:181-­190.

Luo, M. C., Yang, Z. L., You, F. M., Kawahara, T., Waines, J. G., and Dvorak, J. 2007. The structure of wild and domesticated emmer wheat populations, gene flow between them, and the site of emmer domestication. Theor. Appl. Genet. 114:947-­959.

Maccaferri, M., Sanguineti, M. C., Donini, P., and Tuberosa, R. 2003.

Microsatellite analysis reveals a progressive widening of the genetic basis in the elite durum wheat germplasm. Theor. Appl.

Genet. 107:783-­797.

Maccaferri, M., Sanguineti, M. C., Noli, E., and Tuberosa, R. 2005.

Population structure and long-­range linkage disequilibrium in a durum wheat elite collection. Mol. Breed. 15:271-­289.

Martinez-­Perez, E., Shaw, P., and Moore, G. 2001. The Ph1 locus is needed to ensure specific somatic and meiotic centromere association. Nature 411:207.

Meimberg, H., Rice, K. J., Milan, N. F., Njoku, C. C., and McKay, J. K. 2009. Multiple origins promote the ecological amplitude of allopolyploid Aegilops (Poaceae). Am. J. Bot. 96:1262-­1273.

Miflin, B. 2000. Crop improvement in the 21st century. J. Exp. Bot. 51:1-­8.

Mori, N., Kondo, Y., Ishii, T., Kawahara, T., Valkoun, J.. and Nakamura, C. 2009. Genetic diversity and origin of timopheevi wheat inferred by chloroplast DNA fingerprinting. Breed. Sci. 59:571-­578.

Morris, R., and Sears, E. R. 1967. The cytogenetics of wheat and its relatives. Pages 19-­87 in: Wheat and Wheat Improvement.

K. S. Quisenberry and L. P. Reitz, Eds. Am. Society of Agronomy, Madison, WI. Munns, R., and James, R. A. 2003. Screening methods for salinity tolerance: A case study with tetraploid wheat. Plant Soil 253:201-­218.

Nachit, M., Picard, E., Monneveux, P., Labhilili, M., Baum, M., and Rivoal, R. 1998. An international durum wheat improvement programme for the Mediterranean basin. Cahiers-­Agric. 7:510-­515.

Natoli, V. 2008. Identification in durum wheat of QTL involved in mechanisms of adaptation to water stresses in Mediterranean environments. Ph.D. dissertation, University of Turin, Turin, Italy.

Nesbitt, M., and Samuel, D. 1996. From stable crop to extinction?

The archaeology and history of the hulled wheats. Pages 41-­100 in: Hulled Wheats. Proc. First International Workshop on Hulled Wheats. S. Padulosi, K. Hammer, and J. Heller, Eds. Int. Plant Genetic Resources Institute, Rome.

Nsarellah, N. A., Amri, M. M., Nachit, M., Bouhssini, E. L., and Lhaloui, S. 2003. New durum wheat with Hessian fly resistance from Triticum araraticum and T. carthlicum in Morocco. Plant Breed. 122:435-­437.

Okamoto, M. 1957. Asynaptic effect of chromosome V. Wheat Inf. Serv. 5:6. (http://www.shigen.nig.ac.jp/wheat/wis/No5/p6/1.html) Özkan, H., Brandolini, A., Schaefer-­Pregl, R., and Salamini, F. 2002.

AFLP analysis of a collection of tetraploid wheats indicates the origin of emmer and hard wheat domestication in southeast Turkey.

Mol. Biol. Evol. 19:1797-­1801. Özkan, H., Willcox, G., Graner, A., Salamini, F., and Kilian, B. 2011.

Geographic distribution and domestication of wild emmer wheat (Triticum dicoccoides). Genet. Resour. Crop Evol. 58:11-­53.

Pecetti, L., and Annicchiarico, P. 1998. Agronomic value and plant type of Italian durum wheat cultivars from different eras of breeding. Euphytica 99:9-­15.

Peña, R. J. 2000. Durum wheat for pasta and bread-­making. Comparison of methods used in breeding to determine gluten quality-­

14

x

Durum Wheat: Chemistry and Technology, 2nd ed.

related parameters. Pages 423-­430 in: Durum Wheat Improvement in the Mediterranean Region: New Challenges. (Options Méditerranéennes, Ser. A, Vol. 40.) N. Di Fonzo, F. Kaan, and M. Nachit, Eds. Int. Centre for Advanced Mediterranean Agronomic Studies (CIHEAM), Zaragoza, Spain.

Pfeiffer, W. H., Sayre, K. D., and Reynolds, M. P. 2000. Enhancing genetic grain yield potential and yield stability in durum wheat.

Pages 83-­94 in: Durum Wheat Improvement in the Mediterranean Region: New Challenges. (Options Méditerranéennes, Ser. A, Vol. 40.) N. Di Fonzo, F. Kaan, and M. Nachit, Eds. Int. Centre for Advanced Mediterranean Agronomic Studies (CIHEAM), Zaragoza, Spain.

Riley, R., and Chapman, V. 1958. Genetic control of the cytologically diploid behaviour of hexaploid wheat. Nature 182:713-­715.

Salamini, F., Özkan, H., Brandolini, A., Schäfer-­Pregl, R., and Martin, W. 2002. Genetics and geography of wild cereal domestication in the Near East. Nat. Rev. Genet. 3:429-­441.

Salse, J., and Feuillet, C. 2007. Comparative genomics of cereals.

Pages 177-­205 in: Genomics-­Assisted Crop Improvement, Vol. 1.

R. Varshney and R. Tuberosa, Eds. Springer-­Verlag, Dordrecht, The Netherlands.

Salse, J., Bolot, S., Throude, M., Jouffe, V., Piegu, B., Quraishi, U. M., Calcagno, T., Cooke, R., Delseny, M., and Feuillet, C. 2008. Identification and characterization of shared duplications between rice and wheat provide new insight into grass genome evolution. Plant Cell 20:11-­24.

Sears, E. R. 1976. Genetic control of chromosome pairing in wheat.

Annu. Rev. Genet. 10:31-­51.

Simmonds, J., Fish, L., Leverington-­Waite, M., Wang, Y., Howell, P., and Snape, J. 2008. Mapping of a gene (Vir) for a non-­glaucous, viridescent phenotype in bread wheat derived from Triticum dicoccoides, and its association with yield variation. Euphytica 159:333-­341.

Simonetti, M., Bellomo, M. P., Laghetti, G., Perrino, P., Simeone, R., and Blanco, A. 1999. Quantitative trait loci influencing free-­ threshing habit in tetraploid wheats. Gen. Res. Crop Evol.

46:267-­271.

Singh, P. K., Mergoum, M., Ali, S., Adhikari, T. B., Elias, E. M., and Hughes, G. R. 2006. Identification of new sources of resistance to tan spot, Stagonospora nodorum blotch, and Septoria tritici blotch of wheat. Crop Sci. 46:2047-­2053.

Sissons, M. J., and Hare, R. A. 2002. Tetraploid wheat—­A resource for genetic improvement of durum wheat quality. Cereal Chem.

79:78-­84.

Somers, D. J., Fedak, G., Clarke, J., and Cao, W. 2006. Mapping of FHB resistance QTLs in tetraploid wheat. Genome 49:1586-­1593.

Talbert, L. E., Smith, L. Y., and Blake, N. K. 1998. More than one origin of hexaploid wheat is indicated by sequence comparison of low-­copy DNA. Genome 41:402-­407.

Tanno, K., and Willcox, G. 2006. How fast was wild wheat domesticated? Science 311:1886.

Thuillet, A.-­C., Bataillon, T., Poirier, S., Santoni, S., and David, J. L. 2005. Estimation of long-­term effective population sizes through

the history of durum wheat using microsatellite data. Genetics 169:1589-­1599.

Uauy, C., Brevis, J., Chen, X., Khan, I., Jackson, L., Chicaiza, O., Distelfeld, A., Fahima, T., and Dubcovsky, J. 2005. High-­ temperature adult-­plant (HTAP) stripe rust resistance gene Yr36 from Triticum turgidum ssp. dicoccoides is closely linked to the grain protein content locus Gpc-­B1. Theor. Appl. Genet. 112:97-­105.

Uauy, C., Distelfeld, A., Fahima, T., Blechl, A., and Dubcovsky, J. 2006. A NAC gene regulating senescence improves grain protein, zinc, and iron content in wheat. Science 314:1298-­1301.

Valkoun, J. J. 2001. Wheat pre-­ breeding using wild progenitors.

Euphytica 119:17-­23.

Van Slageren, M. W. 1994. Wild wheats: A monograph of Aegilops L. and Amblyopyrum (Jaub. and Spach) Eig (Poaceae). (Wageningen Agricultural University Papers 94-­7). Wageningen Agricultural University, Wageningen, The Netherlands.

Wang, H., Huang, X., Röder, M., and Börner, A. 2002. Genetic mapping of loci determining long glumes in the genus Triticum.

Euphytica 123:287-­293.

Widner, J. N., and Lebsock, K. L. 1973. Combining ability in durum wheat: I. Agronomic characteristics. Crop Sci. 13:164-­167.

Wright, S. I., Vroh, B. I., Schroeder, S. G., Yamasaki, M., Doebley, J. F., McMullen, M. D., and Gaut, B. G. 2005. The effects of artificial selection on the maize genome. Science 308:1310-­1314.

Xu, L., Li, W., Wei, Y., and Zheng, Y. 2009. Genetic diversity of HMW glutenin subunits in diploid, tetraploid and hexaploid Triticum species. Genet. Resour. Crop Evol. 56:377-­391.

Xu, S., and Dong, Y. 1992. Fertility and meiotic mechanisms of hybrids between chromosome autoduplication tetraploid wheats and Aegilop species. Genome 35:379-­384.

Yahiaoui, N., Brunner, S., and Keller, B. 2006. Rapid generation of new powdery mildew resistance genes after wheat domestication.

Plant J. 47:85-­98.

Zaharieva, M., Ayana N. G., Al Hakimi, A., Misra, S. C., and Monneveux, P. 2010. Cultivated emmer wheat (Triticum dicoccon Schrank), an old crop with promising future: A review. Genet.

Resour. Crop Evol. 57:937-­962.

Zhang, L., Yen, Y., Zheng, Y., and Liu, D. 2007. Meiotic restriction in emmer wheat is controlled by one or more nuclear genes that continue to function in derived lines. Sex. Plant Reprod. 20:159-­166.

Zhu, Z., Zhou, R., Kong, X., Dong, Y., and Jia, J. 2005. Microsatellite markers linked to 2 powdery mildew resistance genes introgressed from Triticum carthlicum accession PS5 into common wheat.

Genome 48:585-­590.

Zohary, D., and Feldman, M. 1962. Hybridization between amphidiploids and the evolution of polyploids in the wheat (Aegilops-­ Triticum) group. Evolution 16:44-­61.

Zohary, D., and Hopf, M. 2000. Domestication of plants in the old world: The origin and spread of cultivated plants in West Asia, Europe, and the Nile Valley. Oxford University Press, New York.

CHAPTER 2

Genetics and Breeding of Durum Wheat John M. Clarke Agriculture and Agri-­Food Canada Semiarid Prairie Agricultural Research Centre Swift Current, Saskatoon, Saskatchewan, Canada Enzo DeAmbrogio (Retired) Società Produttori Sementi s.p.a.

Argelato (Bologna), Italy

Genetic improvement of crops has been a key factor in the dramatic increase in crop yields during the past century. Wheat yield increases have been achieved by a combination of increased genetic yield potential and improved agronomic traits such as straw strength and the semidwarf plant habit that has permitted increased application of fertilizers and irrigation water.

Globally, food production must be doubled in the next 50 years because of population growth amid sustainability concerns such as a shrinking land base and diminishing resources such as water (Tilman et al 2002). Climate change, with increases in temperature and, in some regions, increased aridity, will pose further challenges. Yields of crops such as durum will have to be increased by improving the efficiency of use of water and nutrients under increased abiotic stresses, such as high temperature and drought, and potential new biotic stresses caused by new diseases and insect pests. Yields of wheat have been increasing steadily but at a rate below predicted future rates of growth in demand (Reynolds et al 1999).

The continued release of new durum cultivars has improved the end-­use suitability of the crop by keeping pace with changes in consumer preferences, food safety regulations, and processing technology. Concurrent improvements in agronomic traits and disease and insect resistance have served to maintain the economic attractiveness of durum for producers. Durum breeding continues to address new factors, such as consumer concerns about food safety and nutritional benefits. Breeding provides a cost-­effective means to mitigate food safety issues such as the presence of heavy metals, and genetic resistance to diseases can reduce mycotoxins and fungicide residue levels on grain. Public concern about intensive agriculture in areas such as Western Europe means that breeders must also try to reduce the envi-

Raymond A. Hare (Retired) Durum Wheat Improvement New South Wales Department of Primary Industries Tamworth Agricultural Institute Tamworth, New South Wales, Australia Pierre Roumet Institut National de la Recherche Agronomique Unité Mixte de Recherches Amélioration Génétique et Adaptation de Plantes Méditerranéennes et Tropicales Domaine de Melgueil Mauguio, France

ronmental impact of durum production and fit the crop to new management practices such as low-­input and organic systems.

Current breeding programs employ a combination of “conventional” and new molecular tools to facilitate rapid and efficient selection for the many traits required in new cultivars. Our objective in this chapter is to review the global durum breeding effort and to document the results achieved and the objectives for current and future efforts.

Overview of breeding Europe Italy was among the first countries to begin concentrated scientific efforts to improve durum. Almost all of the old Italian durum cultivars were obtained by pure-­line selection within landraces. Among them, the most successful was the cultivar Senatore Cappelli (popularly known as Cappelli), selected by Nazareno Strampelli from a landrace introduced from Algeria.

The release of Cappelli in 1915 raised the average yield in Italy from 0.9 t ha–1 in 1920 to 1.2 t ha–1 at the end of the 1930s (Bozzini et al 1998). Cappelli for many years covered more than 60% of the Italian durum area because of its wide adaptability to different environments and good semolina quality, and it was widely used in many breeding programs around the world (Vallega and Zitelli 1973).

After the end of World War II, most of the newly released cultivars derived from crosses involving Cappelli as one of the parents. Among them, the most successful was Capeiti 8 (equal to Patrizio 6) released by F. Casale. In 1971, Capeiti 8 was the most cultivated Italian cultivar, with 56% of the acreage. The partial replacement of Cappelli by Capeiti 8 was due to the higher

x

15

16

x

Durum Wheat: Chemistry and Technology, 2nd ed.

yield of the latter, but it caused a deterioration of the quality of Italian durum wheat (Vallega and Zitelli 1973). Two other significant cultivars of this period were Appulo, bred by R. Grifoni and widely cultivated because of its high yield, and Trinakria, bred by G. P. Ballatore and interesting because of its quality. The introduction of this first group of cultivars derived from hybridization contributed to a steady increase in durum wheat yield in Italy during the period 1958–1972, which reached a national average of about 2 t ha–1. Nevertheless, during the same period, durum yield was still well below the yield of bread wheat (T. aestivum) (Vallega and Zitelli 1973). One of the reasons for this difference in yield was the high susceptibility of durum to lodging due to its tall straw, exacerbated by the increased usage of fertilizers by farmers.

In 1958, the Laboratory for Plant Genetics and Mutagenesis was established at the Research Center of Casaccia, near Rome.

There, a group of researchers led by G. T. Scarascia Mugnozza and F. D’Amato identified durum mutants induced by irradiation and chemical mutagens. The most interesting mutants, carrying alleles for short straw and earliness, were used to release new cultivars, which, in turn, were used as parents in crosses.

The cultivar Creso, released in 1974, derived from a cross between a Cappelli short-­straw mutant and a durum line from the International Centre for Wheat and Maize Improvement (CIMMYT) having short, stiff straw introgressed from bread wheat (Scarascia Mugnozza 2005). The short and stiff straw of Creso made it possible to use higher amounts of fertilizer and to grow durum in the more fertile areas of central and northern Italy, which increased the national average yield to about 2.5 t ha–1 (Bozzini et al 1998). The yield of Creso under the best agroenvironmental conditions was much higher and was close to the yield of the best bread wheat cultivars. For this reason and because of its durable resistance to leaf rust (Puccinia triticina Eriks.), Creso was widely used in breeding programs in the Mediterranean area.

During the same period, J. Vallega and G. Zitelli were working at the Experimental Institute for Cereal Research in Rome to increase the yield potential of durum by improving lodging resistance, the number of fertile spikelets, and resistance to stem rust (P. graminis Pers.:Pers. f. sp. tritici Erikss. and E. Henn.), leaf rust, and powdery mildew (Blumeria graminis (DC.) Speer).

The approach was to cross Cappelli with plants selected among a segregating population developed by N. Borlaug in Mexico by backcrossing to the North Dakota durum LD 390, resistant to P. graminis, plants derived from the cross between a bread wheat carrying the Norin 10 dwarfing genes and a Mexican durum.

Selected lines derived from these crosses were released as cultivars, while others were crossed with lines derived from the cross Cappelli × Yuma, made to introduce resistance to P. graminis and B. graminis into Cappelli. These last crosses produced several cultivars, of which Valnova, released in 1975, was the most successful (Vallega and Zitelli 1973).

Another public institution involved in durum breeding was the Experimental Station for Wheat at Caltagirone, in Sicily, notable for the release in 1988 of the cultivar Simeto, bred by F. Calcagno. Simeto has been widely cultivated in Italy and the Mediterranean area, mainly because of its good performance in environments prone to drought stress.

Private seed companies have also been involved in durum breeding in Italy. In the company Società Italiana Sementi, founded in 1947, G. Brevedan bred a set of cultivars, among which the most widely cultivated was Duilio, released in 1984.

Società Produttori Sementi Bologna (PSB), founded in 1911, was involved with F. Todaro in pure-­line selection within landraces in the first part of the last century. PSB resumed durum breeding after 1970 with S. Selleri, whose breeding work was continued by E. DeAmbrogio. The cultivars Iride (1996), Levante (2002), and Svevo (1996) are among the 10 cultivars most widely cultivated in Italy. Svevo is noteworthy because it is one of the very few Italian cultivars bred by PSB to meet the requirements of the Italian durum pasta-­production chain, established in 1989 by PSB and Barilla S.p.A., the world leader in pasta manufacture.

All of the public and private institutions mentioned above are still actively involved in durum breeding. Also, the Italian Universities of Bari, Bologna, Sassari, and Viterbo are dealing with durum breeding either directly or through research supporting the breeding work, including genomics, discovery of quantitative trait loci (QTL), and improvement of quality and resistance to stresses (mainly drought) and diseases.

The vast majority of cultivars certified for seed production in Italy by the certifying agency Ente Nazionale delle Sementi Elette were selected in Italy. From 2000 to 2004, the amount of durum certified seed was above 400,000 t per year, decreasing to a minimum of about 233,000 t in 2005, as a result of changes in the European Common Agricultural Policy. The amount of certified seed has been steadily increasing since then. Table 2.1 shows the variation in the amount of certified seed for the 10 most widely grown cultivars from 2002 to 2007. Although certified seed of 125 cultivars was produced in 2007, the top 10 accounted for nearly 65% of all the certified seed.

During the first part of the twentieth century, durum production was localized in marginal areas of France, especially in the southeast. Until the 1960s, production and research programs were concentrated in Algeria, Morocco, and Tunisia, and the milling and pasta industries imported grain from those countries. With decolonization, durum wheat production was increased in France, initially in the south. One cultivar, Bidi 17, which derived from Oued Zenatti (an East Algerian population) by mass selection, was grown. Bidi 17 was tall, with large grains and a weak yellow color index.

In the early 1960s, a public breeding program managed by Pierre Grignac was initiated at Montpellier by Institut National de la Recherche Agronomique (INRA) and Ecole Nationale Supérieure Agronomique. The first crosses utilized mainly Mediterranean landraces as parents. The first registered French cultivars, Montferrier and Agathe, led to an increase in durum production in southern France.

From the mid 1960s, cultivars from North Dakota (such as Lakota, Leeds, and later Cando and Lloyd) were imported for use in northern France. This second gene pool was widely used to improve French durum wheat. Crosses were made to combine the large grain size of the Mediterranean gene pool with the good yellow amber color of the North American gene pool.

By 1970, the first private durum wheat breeding programs appeared, working in close relationship with INRA. Segregating populations or advanced lines were transferred from INRA to

Genetics and Breeding private companies as a way to help the development of durum wheat breeding serve the increasing production area. A third genetic pool was brought in at this time when breeders began using CIMMYT lines as parents.

Crosses between durum and Japanese bread wheat were made by Pierre Grignac to reduce plant height. The development and registration of the first semidwarf cultivar, Durtal, in 1972, was an important step. It permitted the use of intensive-­ agriculture techniques, including fertilizers and higher sowing density, to maximize grain yield. Unexpectedly, Durtal was also a turning point for durum breeding programs in France.

Whereas the cultivar represented a symbol of progress for breeders and producers, industry did not want to use it because of poor end-­use quality.

Cereal chemists quickly demonstrated that the poor quality of Durtal was related to grain protein composition resulting from the use of bread wheat in its parentage. Markers were proposed to characterize gluten strength by differentiating γ-­gliadin bands 42 (unfavorable) and 45 (favorable) (Damidaux et al 1980), which could be used to screen breeding material. Further studies demonstrated that gluten strength was functionally related to the low molecular weight glutenins LMW-­1 and LMW-­2 (Pogna et al 1988). Based on these criteria, breeders screened their nurseries and, step by step, discarded the γ-­42 lines from their breeding material. The process was made even easier by an observed genetic linkage of bronze glume coloration at maturity with presence of γ-­42 (Leisle et al 1981).

The Durtal quality situation had a dramatic effect on durum production in France. Whereas production area had increased steadily from the mid-­1960s to reach 285,000 ha in 1975, with production of 800,000 t, it fell to 100,000 ha and 260,000 t in 1977 and remained stable at this level for six years. To avoid a similar crisis in the future, in 1983, breeders created an association known as GIE Blé dur. It included all of the durum wheat breeders from French private companies and the pasta industry, as well as technical institute delegates. Scientific expertise was provided by INRA. The aim of this group was to develop knowledge of end-­use quality, breeding techniques, and germplasm to contribute to the development of new French durum cultivars.

North America The largest durum breeding program in the United States was founded at North Dakota State University in 1929 (Joppa and Williams 1988), with the first cultivar, Carleton, released in 1943. Cultivars grown before that time were introductions from North Africa, Russia, Greece, and other areas (Joppa and Williams 1988). Cultivars from the North Dakota program dominate in the northern Great Plains production area, with cultivars Lebsock (released in 1999), Mountrail (1998), Pierce (2001), and Ben (1996) being most widely grown. The University of Montana has a small program, affiliated with the North Dakota State program, for the dry areas of the northern Great Plains.

The University of California, Davis also has a small durum improvement program for the southwest desert area.

Several private companies also breed durum in the United States, many centered in Arizona to serve the “Desert Durum” industry of that region. These include Arizona Plant Breeders, Resource Seeds, WestBred LLC (now a division of Monsanto), and World Wide Wheat LLC. In addition, World Wide Wheat LLC, WestBred LLC, and Syngenta now have breeding programs targeted to the northern Great Plains and northwestern United States.

Certified Seed

Simeto Iride Duilio Ciccio Claudio Levante Svevo Creso Colosseo Orobel Top 10’s share of total a Source:

ENSE (no date).

17

As a consequence of the Durtal “crisis,” the main thrust of GIE studies was initially related to development of knowledge of biochemical and genetic determinants of grain quality such as the kernel physical-­appearance factors black point, vitreousness, and brightness and to development of new criteria for screening breeding material. This produced favorable results, and new cultivars and high prices increased durum production to rapidly reach 500,000 ha in 1991. In 1992, “traditional” durum-­producing regions in the European Union were given a special designation, and farmers received a premium price for durum if they used certified seed. As a consequence, production of durum wheat disappeared from central France, which was not a traditional region, and durum surface area dropped temporarily to 220,000 ha. The durum area gradually recovered to reach 460,000 ha in 2007.

TABLE 2.1 Certified Seed of the 10 Most-Cultivated Durum Cultivars in Italy (percent of total certified seed) a Cultivar

x

Year of Release

2002

2003

2004

2005

2006

2007

1988 1996 1984 1996 1998 2002 1996 1974 1995 1999

19.28 2.44 12.30 9.08 1.06 0.00 2.03 7.35 5.72 0.34 59.59

18.81 4.85 11.32 8.37 2.05 0.02 1.74 6.22 5.15 0.64 59.19

19.80 6.39 10.32 7.01 3.16 0.37 2.18 5.92 5.42 1.40 61.96

17.72 8.21 9.64 5.40 4.04 1.38 3.41 4.65 4.18 2.70 61.35

21.40 10.13 8.39 6.16 4.74 2.81 3.26 3.37 3.35 2.40 66.01

19.30 12.18 7.38 5.22 5.06 4.56 2.80 2.77 2.65 2.53 64.45

18

x

Durum Wheat: Chemistry and Technology, 2nd ed.

The earliest records of durum wheat breeding in Canada are from 1928, but little effort was put into it until the early 1950s (Knott 1995). Breeding effort was increased at that time to combat the epidemic caused by stem rust race 15B in North America.

Breeding programs were expanded at the Agriculture and Agri-­ Food Canada Research Station at Winnipeg, MB, under D. Leisle and later at the Research Station at Regina, SK, with E. A. Hurd and at the University of Saskatchewan at Saskatoon with D. R. Knott. The Regina program was subsequently transferred to Swift Current, SK, in 1968 (Knott 1995). The first Canadian-­ developed durum cultivar was Stewart 63, registered in 1963 (Knott 1963). Canadian durum production before that was based on cultivars introduced from the United States, such as Mindum, Carlton, Stewart, and Ramsey and the Algerian cultivar Pelissier.

It was soon recognized that the only possibility for durum production to increase in Canada was to develop export-­markets opportunities. This in turn required better end-­ use quality, particularly gluten strength. Hercules, released in 1969 (Leisle 1970), was the first cultivar with gluten strength acceptable to the Italian market, and for many years it was the statutory standard for end-­use quality in the Canadian cultivar-­registration system. The cultivars Wakooma and Kyle (Hurd et al 1973, Townley-­Smith et al 1987) combined this improved quality with higher grain yields, and Canadian durum production increased to 2–2.5 million ha annually by 1990. Canada is currently the dominant global exporter of durum. Kyle is the most successful Canadian cultivar to date, capturing a maximum of 78% of Canadian durum production area in 1999. At peak production, Kyle thus constituted more than 50% of the world durum trade.

Currently, Canada has two major durum wheat breeding programs. The largest is at the Semiarid Prairie Agricultural Research Centre of Agriculture and Agri-­Food Canada (the federal ministry of agriculture) at Swift Current, SK, with leadership transferred from J. M. Clarke to A. K. Singh in 2008. The other is at the Crop Development Centre of the University of Saskatchewan at Saskatoon, under C. J. Pozniak. Both programs are funded by a mix of public and private sources, the latter coming from farmers through a voluntary levy on grain sales and from private companies. Durum breeding programs at the Cereal Research Centre of Agriculture and Agri-­Food Canada at Winnipeg and at Saskatchewan Wheat Pool, a grain handling company, were terminated in 1995. Only about 10% of the durum area in Canada is sown with certified seed, and there is no end-­ point royalty system, so private involvement in breeding is minimal at this point. Syngenta and World Wide Wheat LLC have small durum breeding programs in western Canada.

Australia The Australian durum wheat industry is a relatively recent development when compared with the major international producers. Up until the late 1970s, the annual harvest was about 10,000 t. Since that time, production has increased to an average harvest of about 700,000 t in good seasons, with crops grown in all mainland states, the primary focus being in northern New South Wales and South Australia. Most of the current Australian durum is exported into high-­quality markets around the world.

William James Farrer was Australia’s first durum wheat breeder. After hearing from M. A. Carleton, a very helpful and long-­term breeding colleague in the United States, that a durum industry based on Russian cultivars had been successfully established in the midwestern states, Farrer felt that it would be profitable to attempt the same in Australia. Farrer believed that these wheats could be of great value to Australia, for the reason that, on the one hand, some varieties produce payable yields in localities where the climate is too dry for bread wheats; and, on the other, because there are varieties which are so resistant of rust as to be able to withstand the rusty conditions of our coastal climate, and produce crops of grain in places where rust appears to have made impossible the cultivation of bread wheats

(Farrer 1903). Despite a substantial durum breeding effort, Farrer did not succeed in convincing Australian farmers to grow durum wheat commercially. While durum wheat displayed certain production advantages over other cereals, the Australian market was not ready for durum grain at that time. Archer Russell, in his biography of Farrer, suggested that the huge success of Farrer’s bread wheats, in particular Federation, a high-­ yielding, quick-­maturing cultivar of good quality, considerably lessened the interest in Farrer’s durums (Russell 1949).

Twenty-­eight years elapsed after Farrer’s death (1906) before the New South Wales (NSW) Department of Agriculture breeders, especially Steven Macindoe, followed by Ted Matheson and Bill Single, championed the durum-­breeding cause (Macindoe and Walkden Brown 1968). A small-­ scale durum breeding program was commenced in 1934 by the NSW Department of Agriculture at the New England Experiment Farm, Glen Innes.

Discontinued during the war years, the program recommenced in 1948 and was transferred to the Agricultural Research Centre, Tamworth, in 1958. Dural, the first Australian pasta-­ quality durum cultivar was released in 1955, in response to a growing but small demand for durum pasta in cans. The next durum released, Duramba, in 1968, was Australia’s first semidwarf wheat. Since 1968, a modest number of cultivars have been released by NSW Agriculture (now NSW Department of Primary Industries) from the Tamworth Agricultural Institute (Durati in 1977, Kamilaroi in 1982, Yallaroi in 1987, Wollaroi in 1993, Tamaroi in 1997, Gundaro in 1998, EGA Bellaroi in 2003, Jandaroi in 2006, and Caparoi in 2009). In 2003, the Waite Agricultural Institute, University of Adelaide, released Kalka, a boron-­tolerant cultivar suitable for the South Australian production region. The later cultivars now completely dominate Australian production from Queensland through New South Wales and South Australia to Western Australia. Cultivars must provide growers with a commercially viable alternative to other agricultural enterprises, while at the same time offering processors grain quality that is highly competitive in a strong and discerning international market. Current Australian cultivars produce grain that yields semolina with high yellow pigment levels and a high protein content that exhibits strong rheological properties.

The public programs are funded by the host organizations and by grants from the Grains Research and Development Corporation, which distributes grower research levies (based on

Genetics and Breeding farm gate value) and an equivalent matching allocation from the Australian government.

Four Australian private wheat breeding organizations (Longreach Plant Breeders, Nuseed, Heritage Seeds, and GrainSearch) are selecting durum cultivars but have not made releases to date. The enactment of the Plant Breeder’s Rights law has stimulated private investment in wheat breeding, but the Australian durum industry remains too small to support a full-­ scale private program from seed and end-­point royalties.

Other Programs Durum breeding began at CIMMYT in Mexico under Nobel Laureate Norman Borlaug in 1965 (Pfieffer and Payne 2005).

The program is a major source of improved germplasm used by other durum breeding programs worldwide. Direct selections from CIMMYT lines have been released as cultivars in many countries, and CIMMYT germplasm appears in the pedigrees of many other cultivars worldwide. A joint program of CIMMYT and the International Centre for Agricultural Research in the Dry Areas (ICARDA) in Aleppo, Syria, has strong links to national durum breeding efforts in West Asia and North Africa.

CIMMYT and ICARDA also play an important role in the training of breeding personnel from less-­developed nations. The CIMMYT Mexico program is under the leadership of Karim Ammar, and the ICARDA program is led by Miloudi Nachit.

Turkey has been for many years a large producer of durum wheat, much of it consumed domestically. Breeding efforts started in earnest in 1967 under an agreement with the Rockefeller Foundation, with CIMMYT and Oregon State University providing germplasm and training (Zencirci et al 1996). CIMMYT still maintains a presence in Turkey. Breeding efforts increased productivity and quality, but significant challenges remain because of low adoption of new technology by farmers and inadequate research funding and infrastructure (Ozberk et al 2005).

In addition there are numerous smaller durum breeding programs, both public and private. These are located in Spain, Germany, Austria, India, and North African nations such as Morocco.

Breeding objectives Yield and Agronomic Traits Increased grain yield and improvement of other agronomic traits such as straw strength, dwarf height, and time to maturity are important objectives for breeders. The grain yield of durum must be increased to keep pace with yield improvements in other crops so that producers see durum as an economically viable cropping choice. Progress in yield improvement is discussed later.

The most recent Canadian cultivars are considerably shorter and stronger-­strawed than Kyle (Townley-­Smith et al 1987), which was the predominant cultivar until 2002. AC Avonlea (Clarke et al 1998b), Strongfield (Clarke et al 2005a), and AC Morse are of intermediate height, and AC Navigator (Clarke et al 2000) and Commander (Clarke et al 2005b) are of semidwarf stature. These developments have been in response to changes in farm production practices, particularly the change to direct-­ combine harvesting, and some production of durum under ir-

x

19

rigation. Most cultivars from the North Dakota program are of intermediate height (Elias and Manthey 2005), while those grown in the desert southwestern United States are semidwarfs.

As noted above, strong-­strawed semidwarf cultivars have permitted production of durum in central and northern Italy and have facilitated intensive management of durum in France. All Australian durum cultivars are semidwarfs, based on Rht1.

The dwarfing genes (e.g., Rht1, gibberelli-­acid-­insensitive) have been shown to have undesirable pleiotropic effects such as reduced grain protein concentration, low test weight (McClung et al 1986), and short coleoptile length. Protein concentration can be amended through nitrogen management, and semidwarfs with high test weight have now been developed (e.g., Clarke et al 2000). In Australia, breeding populations are being converted to the gibberellic-­acid-­sensitive height-­controlling gene Rht18.

This gene allows for semidwarf height and coleoptiles of normal length, an advantage in establishing crops under dryland cultivation, particularly when sowing moisture is limited (Condon et al 2004).

Resistance to Abiotic Stresses Drought is a common feature of many durum-­growing areas, including the Mediterranean region, Australia, Canada, and the western United States, especially when the crop is approaching maturity. Breeders therefore attempt to develop high-­y ielding cultivars that are able to maintain acceptable yield when water is limited rather than very “drought-­resistant” cultivars that may not have good yield potential under favorable moisture regimes.

Early maturity can partially avoid terminal drought stress, but this must be balanced by the negative impact of earliness on yield. Replicated trials across locations and years are an effective means of selection for yield potential in dry and variable environments (Hurd 1974). There is also hope that some of the QTL associated with water-­use efficiency can be validated in different genetic backgrounds and then, when suitable markers are available, used in marker-­assisted selection. Australian research shows that differences in carbon isotope discrimination can reveal genetic variability for stomatal conductance (movement of water vapor out of the leaf) and hence water-­use efficiency (Condon et al 2004). Durum genotypes also display useful genetic variability for osmotic adjustment, the control of osmotic pressure, and therefore cellular turgor pressure against increasing water deficits (Morgan et al 1986).

In areas where durum is grown as a cold-­season or facultative crop, cold stress at flowering time can induce a degree of sterility. Selection for resistance to this stress is not easy because it does not appear every year. Frost stress during the vegetative period is more common in northern Italy and in France but, again, is not present every year. Nearly all of the durum cultivars grown in Italy are spring type but autumn-­sown, which is generally an advantage in terms of yield, but this method is more risky in the case of early cold stress. It is possible to transfer the cold resistance of winter durums into spring-­type cold-­susceptible cultivars, but special facilities are required for selection because selection in the field is very unreliable under Italian conditions.

This is another trait that should be investigated for the possibility of marker-­assisted selection.

20

x

Durum Wheat: Chemistry and Technology, 2nd ed.

High concentrations of soil boron (B) constitute a significant production problem in many parts of the world, particularly in low-­rainfall regions and irrigation areas (Rashid and Ryan 2004). Low B accumulation in plant tissues is associated with improved biomass development and grain yield where soil B levels are elevated. Alleles at the Bo1 locus reduce the uptake of B in durum wheat (Jamjod 1996). Boron tolerance is an essential adaptational trait in the breeding of cultivars intended for southern Australia.

Durum wheat is poorly adapted to acid soil conditions (pH 4–5) where high available-­a luminum concentrations (Al3+) are a feature. A large proportion of the wheat belt in Western Australia and smaller areas in southeastern Australia are characterized by acid soils. Recent screening research has uncovered durum accessions that have tolerance to high Al3+ concentrations at pH 4.2, comparable to levels expressed by tolerant bread wheats.

Further study is required to determine the mode of inheritance at the tetraploid level, although there is an expectation that the mode will be simple, i.e., one or two loci. Tolerance gene(s) are being introduced into breeding populations with the expectation of providing growers in the Australian acid-­soil areas the option to produce high-­value durum grain.

Resistance to Diseases and Insects Diseases that cause mycotoxin contamination of grain are of primary interest to the food industry and consumers. Producers therefore wish to minimize such diseases to maintain market value. Diseases that affect photosynthetic leaf area or damage stem conductive tissue reduce both grain yield and quality. For example, test weight and kernel weight reductions of up to 7 and 24%, respectively, were caused by leaf rust when susceptible durum cultivars were not protected by fungicide treatment (R. P. Singh et al 2004). Breeding for disease resistance is a cost-­effective way to reduce such economic losses and serves to reduce the application of costly fungicides. Genetic resistance to both diseases and insects is a desirable goal in low-­i nput production systems, which are found in much of the area in which durum is produced. The cultivar development strategy for durum in Canada emphasizes genetic resistance to insects and diseases to meet the strict requirements of registration of cultivars (Clarke et al 1998a). Adequate resistance to all field pathotypes of the three wheat rusts is an essential requirement for release in Australia. For further discussion of the diseases and insects of durum wheat, see Chapters 4 and 5, respectively.

Diseases

The major diseases of durum in the northern United States and Canada include leaf rust, stem rust, common bunt (Tilletia laevis J.G. Kühn and T. caries (DC.) Tul. & C. Tul.), loose smut (Ustilago tritici (Pers.) Rostr.), Fusarium head blight (FHB) (caused by various Fusarium spp.), and leaf-­spotting diseases caused by several organisms. Bunt resistance tends to be very good in North American durum, but loose smut resistance is variable. Leaf and stem rust caused major economic losses in the past, but development of resistant cultivars and on-­going vigi-

lance by cereal pathologists to identify changes in the pathogenic races of these diseases have virtually eliminated yield losses.

However, new rust races pose a threat to durum in North America. The recently identified leaf rust race BBG/BN in Mexico (Herrera-­Foessel et al 2005) and France (Goyeau et al 2006) is virulent on all Canadian durum cultivars, so work has started to introduce resistance. Markers have been identified for some effective resistance genes (Herrera-­Foessel et al 2007), which will facilitate breeding for resistance. A new stem rust race, TTKS (popularly known as Ug99), detected in Uganda (Pretorius et al 2000) is virulent on current major durum cultivars, so incorporation of resistance genes would be required if the race spreads to North America.

Leaf rust is a major disease of durum in the Mediterranean basin, including Italy. It is induced by races having different virulence patterns in different areas of the country (i.e., pathotypes). Breeders at PSB artificially inoculate breeding nurseries with pathotypes having complementary virulence patterns. In this way, there is a fairly high level of disease every year, and the selected plants are expected to be resistant to a range of pathotypes; therefore, their resistance should be reliable and possibly also durable. Stem rust is not a problem in Italy; the current cultivars are usually able to avoid significant yield reduction because of their earliness.

Unlike the case in common wheat, not many genes for leaf rust resistance are known in durum and therefore it is not easy to pyramid them with marker-­assisted selection to provide durable resistance. Of great interest is the recent identification of a major QTL controlling the durable resistance of the durum cultivar Creso, which has lasted for more than 30 years in the Mediterranean area (Maccaferri et al 2008a). Markers linked to this QTL are available, so there is the possibility of using the marker-­assisted selection approach, even though it might be advisable to introgress the QTL into cultivars with some degree of resistance.

Stripe rust (P. striiformis Westend. f. sp. tritici), also known as yellow rust, is an important and destructive disease in many areas of durum production, such as the Middle East and North Africa (Mamluk 1992). Historically, stripe rust has been widespread in the northwestern United States, which often spreads into southwestern Alberta, Canada (Line 2002). Recently, however, the disease has spread to the south-­central United States and the Great Plains area (Chen et al 2002), and there have been occurrences in the Canadian durum production area. To date, no effort has been put into breeding for stripe rust resistance in Canada. In Italy, stripe rust does not frequently cause significant damage because it is not favored by the climatic conditions and also because resistance appears to be widespread in Italian cultivars (Bozzini et al 1998).

All three wheat rusts have the potential to cause significant economic losses to durum production in Australia. Australian wheat breeders have been serviced by the Australian National Cereal Rust Control Program from the Plant Breeding Institute at the University of Sydney since the mid-­1970s. All public and private breeding materials can be tested for resistance at both the seedling and adult plant stages against a selection of appropriate pathotypes for each rust species. This program provides advice and germplasm sources of new effective resistance genes

Genetics and Breeding to all breeders. All Australian durum cultivars are resistant to known Australian field pathotypes of stem, leaf, and stripe rusts. Preemptive resistance breeding against exotic virulence is conducted by ICARDA as part of a collaborative program with Australian breeders. Preliminary genetic studies suggest that Sr13 together with other Sr genes provide adequate protection against stem rust in Australia (Bhavani 2006). Adult plant resistances appear to be the prime source of acceptable resistance to both leaf rust and stripe rust in Australia (Bariana et al 2007).

Fusarium head blight in durum wheat has become a concern in recent years because of its negative effects on pasta quality attributes such as color (Dexter et al 1997) and the food safety issues resulting from fungal mycotoxins. Many countries have recently tightened regulations concerning the content of the mycotoxin deoxynivalenol (DON) in grain and food products. For example, European Union regulation 856/2005 sets the maximum level of DON allowed in durum at 1.750 µg kg–1. FHB epidemics have occurred in many durum-­growing regions in the world, including the United States, Canada, and northern Italy.

Twenty years ago, FHB resistance was not a breeding objective in the United States (Joppa and Williams 1988), indicating how quickly the disease has become a significant production constraint. This disease is an occasional problem in Australia when rains and high humidity are persistent following anthesis.

Recent work on breeding for resistance to FHB, particularly at North Dakota State, is a reflection of the constraints to production in the northeastern Great Plains area of the United States. No highly effective resistance to FHB has been identified in durum (Ban et al 2005, Elias and Manthey 2005), so most research has focused on the transfer of resistance from other tetraploid wheat species (Chen et al 2007) or from T. aestivum (Elias and Manthey, 2005). T. dicoccoides has also been used as a potential source of resistance for transfer to durum. Stack et al (2002) identified a durum line, Langdon (DIC-­3A), with T. dicoccoides chromosome 3A and a moderate level of resistance. T. dicoccoides is also a potential source of resistance to other diseases, such as stripe rust (Uauy et al 2005). In Italy, selection for resistance started recently, making use of artificial inoculation with virulent and toxigenic strains of Fusarium spp. (DeAmbrogio et al 2007a). Selection for resistance to FHB also tends to reduce DON content (DeAmbrogio et al 2007b). In Canada, modest improvement in FHB resistance was achieved through selection within the Canadian germplasm pool (Clarke et al 2010).

Crown rot (CR) (caused by F. pseudograminearum O’Donnell & T. Aoki) is the only major disease of durum not under effective control in Australia. CR can result in up to 50% yield loss when infection is severe. Crop rotation involving nonhost species (e.g., broadleaf) is the only widely used and effective control measure at present. The complete breakdown of infected crop residues during the nonhost phases of the rotation reduces inoculum to negligible levels.

Leaf spot diseases—­tan spot, caused by Pyrenophora tritici-­ repentis (Died.) Drechsl., anamorph Drechslera tritici-­repentis (Died.) Shoem.; Stagonospora nodorum blotch, caused by Phaeosphaeria nodorum (E. Müll.) Hedjaroude, anamorph Stagonospora nodorum (Berk.) Castell. & E.G. Germano; and Septoria tritici blotch, caused by Mycosphaerella graminicola (Fuckel) J. Schröt. in Cohn, anamorph Septoria tritici Roberge—­

x

21

are prevalent in the Canadian and northern U.S. production areas. Tan spot may cause yield reductions as high as 50% (deWolf et al 1998) through factors such as reduced kernel weight (Fernandez et al 2002). Tan spot inoculum is also prevalent across Australia, especially following moist growing seasons and where stubble-­retention agronomy is practiced. Identification of resistance genes for P. tritici-­repentis races (P. K. Singh et al 2006a, 2008) will facilitate breeding for tan spot resistance.

Potential for resistance to tan spot and other leaf spot diseases exists in durum and related tetraploid germplasm and can be exploited (P. K. Singh et al 2006b,c). In Italy, S. tritici causes yield reductions when climatic conditions are favorable to the spread of the disease. Breeding for resistance to this disease is just beginning in Italy.

Ergot (Claviceps purpurea (Fr.:Fr.) Tul.) poses a health risk from alkaloids in the fungal sclerotia that contaminate grain.

There is evidence of genetic variation for resistance in durum wheat, with one genotype appearing to have good resistance (Menzies 2004). Crosses with a resistant durum cultivar indicate that the trait is heritable and could be selected in a breeding program (J. M. Clarke and J. G. Menzies, unpublished).

Powdery mildew is fairly frequent in northern and central Italy, where it can reduce yield to some extent, whereas, in the south, the progress of the disease toward the top of the plants is generally prevented by early-­season high temperatures and little yield reduction results. In the near future, it will be possible to take advantage of the QTL recently shown to be responsible for the durable resistance of the cultivar Claudio (M. Maccaferri and E. DeAmbrogio, unpublished).

Soil-­borne cereal mosaic virus was first reported affecting wheat crops in Italy in 1960. It is fairly widespread and can reduce yield by up to 70% (Vallega et al 2003). Resistance to this virus is present in durum cultivars grown in Italy, and recently a major QTL controlling resistance was identified (Maccaferri et al 2011). This offers the possibility of using marker-­assisted selection in the near future.

Insects

The wheat stem sawfly, Cephus cinctus Norton, has until recently been the only major insect pest in the Canadian durum-­ production area. Sawfly larvae cause yield loss by feeding on the inside of the stem wall, reducing translocation of nutrients to the kernel, and by girdling the stems at ground level just before harvest, which causes them to fall over. The insect cannot be effectively controlled by insecticides because it spends most of its lifecycle inside the plant. Solid stems provide a physical barrier that reduces sawfly larvae damage (Kemp 1934). Currently, no solid-­stem durum cultivars are grown in Canada, but breeding efforts are underway as a results of recent increases in the incidence of sawfly. The sawfly is also a pest in North Africa.

Recently the wheat midge, Sitodiplosis mosellana (Gehin) (Diptera: Cecidomyiidae), has become a problem in at least 50% of the Canadian durum-­production area. The midge larvae begin feeding on the developing wheat grain soon after anthesis and cause yield and quality losses. Damaged kernels produce semolina with high speck counts, and gluten strength is reduced in the case of nearly mature kernels damaged by feeding. A natural

22

x

Durum Wheat: Chemistry and Technology, 2nd ed.

antibiosis mechanism to combat the wheat midge was identified in bread wheat (Ding et al 2000). A survey of diverse durum germplasm, however, found no antibiosis (Lamb et al 2001), so the antibiosis mechanism has been transferred from common to durum wheat.

Sunn pests (Eurygaster spp.) are widespread in the Mediterranean region and, like the midge, reduce both grain yield and dough strength, the latter through secretion of proteolytic enzymes into kernels. There is evidence of variable resistance to the pest in common wheat (Kinaci et al 1998, Sivri et al 2002) and the potential for selection in breeding programs using specific glutenin fractions affected by the protease (Sivri et al 2002).

The Hessian fly, Mayetiola destructor (Say), is found throughout many durum-­growing areas such as North America, North Africa, and southwestern Europe (Nsarellah et al 2005). The pest causes substantial losses in North Africa (Nsarellah et al 2005), but it is generally not a problem in North American durum-­ production regions. There are many biotypes of the insect, and single resistance genes are frequently overcome by insect mutation. Genes for resistance can be found in durum, common wheat, and wild relatives, and combinations of these are being deployed in areas prone to Hessian fly damage (Nsarellah et al 2005). Several DNA markers have been found that will facilitate this effort (Dweikat et al 2002, Liu et al 2005).

Geographic isolation and quarantine have kept all major insect pests of wheat out of Australia. To reduce the impact of a pest incursion, Australia public breeders are undertaking preemptive resistance breeding with ICARDA against wheat sawfly and Hessian fly.

Nematodes

The root lesion nematodes (Pratylenchus thornei, P. neglectus) and cereal cyst nematode (Heterodera avenae Woll.) are potential threats to durum in Australia. However, genetic resistance together with crop rotations currently provide adequate protection.

End-­Use and Nutritional Quality Pasta is made of semolina and water, so high and stable quality of the raw material, durum wheat, is very important. From the miller’s point of view, quality means all of the traits that favor a high semolina extraction rate, such as high test weight and low ash content in semolina. For the pasta industry, quality means high protein concentration and gluten strength and yellow pigment, which will deliver to the consumer pasta with an attractive appearance and good cooking and eating quality.

Breeding has mainly focused on processing quality parameters, but there is now interest in the improvement of nutritional properties such as content of essential micronutrients and increased levels of resistant starch.

Grain protein concentration has long been a fundamental determinant of the value of durum for pasta manufacture (Dexter and Matsuo 1977) and remains so with newer processing techniques such as high-­temperature drying (D’Egidio et al 1990, Dexter and Marchylo 2001). Pasta manufacturers typi-

cally require a grain protein concentration of 13% (db), ensuring 12% in semolina and pasta to satisfy the textural requirements for cooked pasta. Although protein concentration can be readily increased by nitrogen fertilizer application, it can also be increased genetically. Efficiency of nitrogen use is becoming important due to legislated restrictions on rates of nitrogen fertilizer application, to reduce contamination of surface and subsurface waters, as well as to the high cost of nitrogen fertilizers.

Production of “organic” or “biological” wheat has also prompted interest in exploiting the genetic improvement of grain protein concentration. However, as yet no effective tools have been developed for selection for this trait. Protein concentration can be readily measured on whole or ground grain using near-­infrared (NIR) spectroscopy, but the genetic gain from selection tends to be low because of the large effect of environment on expression of protein.

High grain protein concentration is a requirement for durum cultivar registration in Canada and Australia to maintain protein concentration at as high a level as possible under low-­input production. In Canada, lines evaluated in registration trials must have a protein concentration at least numerically equal to the mean of the check cultivars. The Canadian cultivars AC Avonlea (Clarke et al 1998b) and Strongfield (Clarke et al 2005a) show increased grain protein concentration relative to previous cultivars. The high-­protein-­achieving cultivar EGA Bellaroi has set the registration benchmark for Australian releases.

The yellow pigment concentration of semolina and pasta has grown in importance with increased global competition in pasta marketing (Dexter and Marchylo 2001). Plant pigments are also of interest in relation to antioxidant properties (Miller et al 1996) and provitamin A content (Graham and Rosser 2000).

Additionally, lutein, the major component of durum pigment, may play a role in the prevention of age-­related macular degeneration (Olmedilla et al 2001). Yellow pigment concentration is complexly inherited, but heritability is quite high, so breeders can readily make genetic gains (F. R. Clarke et al 2006).

Pigment concentration can be measured in breeding programs using NIR/visual reflectance spectroscopy (McCaig et al 1992).

Marker-­assisted selection for pigment will soon be possible as a result of recent work identifying loci controlling major components of the pigment synthetic pathway (Pozniak et al 2007, Zhang and Dubcovsky 2008).

The target for pigment concentration has been raised in Canada to meet customer requirements. It now ranges from that expressed by AC Avonlea (8.3 ppm) to that of Commander (9.8 ppm) (Clarke et al 2005b). The pigment concentration of Kyle, the major cultivar from 1988 to 2003, was about 20% lower than that of AC Avonlea. Yellow pigment levels close to or higher than that of EGA Bellaroi are now regarded as the minimum standard for future Australian releases.

Gluten strength is an important factor in pasta manufacture and cooking quality (Feillet and Dexter 1996). Gluten strength predictors such as sodium dodecyl sulfate (SDS)–sedimentation volume and mixograph value, which are used in breeding programs, are moderately heritable (Bratten et al 1962, McClung and Cantrell 1986). Gluten strength targets for Canadian durum have been raised in recent years. Expressed as gluten index, the range for “conventional” durum cultivars is from that of AC

Genetics and Breeding Morse (61) to that of Strongfield (69), and for the “strong gluten” cultivars, a target similar to that of Commander (>90) is preferred. No specific cultivar quality parameters are required for release of cultivars in the United States. The gluten strength of cultivars produced in the desert Southwest is 65–95, while those from the northern plains show a broad range of strength. All Australian durum cultivars are regarded by the market as strong to extra strong, expressing strong, stable rheological properties and high gluten indexes (70–95).

High semolina yield is important to the economics of durum milling. The trait is difficult to select for because of the high labor requirement for milling of samples. Chaurand et al (1999) reported that genetic differences in semolina yield exceeded those for environments and suggested that breeding for semolina yield and other milling properties was worthwhile.

Ripetti-­Ballester et al (2000) demonstrated that NIR spectroscopy could be used to predict the milling yield of whole-­g rain samples. A calibration developed from 10 cultivars grown in four trials over two years explained a large portion of the variation (r 2 = 0.93, n = 72). Validation of the equation in eight samples grown in a different year gave r 2 = 0.79. In Canada, the potential for NIR spectroscopy to predict semolina yield was investigated in a large set of genotypes (McCaig et al 2005), and the resulting equation explained, on average, less than 60% of the variation for semolina yield. Similarly in Australia, NIR spectroscopy was used to predict semolina yield from whole grain in a set of 943 durum wheat lines over five years. The calibration was reasonable (r 2 = 0.75, standard error of cross validation = 1.01), while a validation of the equation could explain only 59% of the variation in semolina yield (Sissons et al 2006).

Breeders already utilize NIR instruments for measurement of protein and color (Clarke et al 1998a), so calibrations for additional parameters do not add further cost to the screening of breeding lines.

Sissons et al (2000) evaluated the single-­kernel characterization system for prediction of semolina yield and found that, although it predicted a relatively small portion of the variation (r2 = 0.25), it might have some potential to eliminate the lowest-­y ielding lines in early generations of a breeding program.

Another approach to selection for semolina yield would be discovery of DNA markers that could be applied to breeding material. Several such studies have been reported in common wheat (Parker et al 1999, Breseghello and Sorrells 2006). Many of the reported QTL reside on the A and B genomes and so should be investigated in durum wheat.

Semolina ash content is important in some markets such as the European Union (Troccoli et al 2000) and is thus an important selection criterion in Europe and in countries that export to Europe. Ash is affected by both genetics and environment (Chaurand et al 1999). For example, kernel ash is greater in environments with high crop transpiration than in those with low transpiration (Araus et al 1998).

The area of nutritional quality is of increasing interest to consumers. This is reflected in the increased commitment of government research funding to nutrition and food safety issues. Nutritional quality also links with medical and health sciences research on promotion of good health through appropriate diet. Interest spans topics such as the content of vi-

x

23

tamins and antioxidant phytochemicals (i.e., sterols, tocols, alkylresorcinols, folates, phenolic acids) and the “glycemic index” (Jenkins et al 1981), which provides a relative measure of the rate of release of sugars into the blood stream. High levels of amylose (“resistant starch”) would reduce the glycemic index and potentially reduce the potential for development of diabetes. High levels of resistant starch may also contribute to improved human colonic health, either mediated through the action of short-­chain fatty acids or resulting from its prebiotic effects (Nugent 2005).

Watanabe and Miura (1998) reported genotypic variation in the amylose content of durum, based on a survey of 665 accessions. Most of the lines had an amylose content in the normal range of 25–35%, but a few exceeded 40%. These results could not be confirmed in subsequent research, but breeders are exploiting other genetic variations to produce high-­a mylose genotypes for quality and nutritional evaluation as well as trying novel approaches to increase amylose content. For example, they have made crosses with common wheat mutants lacking the starch granule proteins, SGP-­1, which are starch synthases bound to starch granules in wheat endosperm (Yamamori et al 2000). The progeny are then backcrossed to durum wheat, and lines lacking the SGP-­A1 and SGP-­B1 proteins can be selected (Lafiandra et al 2006). Durum wheat deficient in both SGP-­A1 and SGP-­B1 proteins has significantly higher amylose content than ordinary durum. Another approach is to make use of targeting-­induced local lesions in genomes (TILLING) (Comai and Henikoff 2006), a reverse genetic strategy allowing the detection of induced-­point mutations affecting the ratio of amylose to amylopectin in individuals within a mutagenized population.

Other Traits Concentration of the nonessential heavy metal cadmium in staple foods such as the cereal grains is a long-­standing concern (Wagner 1993). Discussions under the FAO Codex Alimentarius seek to establish international standards, and the European Union in 2003 established a limit of 0.2 µg g–1 in grains. Soils of the North American durum-­production area contain elevated natural amounts of cadmium deriving from the rock that formed the soils.

Genetic variation for grain cadmium concentration was found in durum, with low concentration controlled by a single dominant gene (Clarke et al 1997). Incorporation of this highly heritable trait into cultivars reduces average grain cadmium to levels well below proposed international limits, averaging 50% lower than that in conventional durum cultivars. The allele for low concentration appears to be specific for cadmium; it has generally no effect or inconsistent effect on concentrations of other ions and does not appear to detrimentally affect any major economic traits (Clarke et al 2002). A DNA marker (Penner et al 1995) is used to select for low grain-­cadmium concentration in Canadian durum breeding programs. The low-­cadmium cultivar Strongfield (Clarke et al 2005a) entered commercial production in Canada in 2006 and covered 60% of the seeded area in 2010 (Table 2.2). All new durum cultivars registered in Canada must be low-­cadmium types.

24

x

Durum Wheat: Chemistry and Technology, 2nd ed.

Soil salinity causes significant reductions in plant productivity and consequent economic losses associated with reduced grain yield and quality of the agricultural crops (Pitman and Läuchli 2002). More than 6% of the world’s land is affected by either salinity or sodicity (FAO 2000). Irrigation systems are particularly prone to salinization, with about half the existing irrigation systems of the world now under the influence of salinization or waterlogging, due either to low-­quality irrigation water or to excessive leaching and the subsequent rising of water tables (Szabolcs 1994). A proportion of the durum wheat production areas coincide with salinized land across the world, in particular, in Australia.

The serious impact of current and future salinization on agricultural production can be restricted or even ameliorated by changes in farming and irrigation management techniques. In the case of dryland salinity, these include restricting the amount of water passing beyond the rooting zone by reintroducing deep-­ rooted perennial species. These deep-­rooted species dry the soil profile and ensure that the water table carrying salt does not reach the soil surface, where water evaporates and salt is deposited in ever-­increasing amounts. Genetic salt tolerance will be required for the “de-­watering” species, but also for the annual crops (including durum) that follow, because salt will be left in the soil when the water table is lowered.

Significant genetic variation for salt tolerance exists in durum wheat, similar in magnitude to that of salt-­tolerant bread wheats. Na+ accumulation in leaf tissues of tolerant genotypes can be one-­tenth the concentration in nontolerant genotypes, and the tolerant genotypes have the highest K+ concentration and greatest K+/Na+ discrimination. The low-­Na+ accessions can exclude Na+ over a wide range of salinity levels and produce greater biomass and grain yield in saline soils. The Na+-­ exclusion trait eliminates sodium toxicity symptoms and premature leaf senescence and allows the flag leaf to remain alive until maturity (Husain et al 2003). Two interacting dominant genes (Munns et al 2003), designated Nax1 and Nax2 (Lindsay et al 2004, Huang et al 2006), condition the Na+-­exclusion trait in durum.

TABLE 2.2 Trends in Grain Protein Concentration, Pigment Concentration, and Alveograph Work Input of Canadian Durum Cultivars and Market Share in 2011

Cultivar

Hercules Kyle Plenty AC Melita AC Morse AC Avonlea AC Navigator Napoleon Strongfield Commander CDC Verona

Percent of Durum Year of Protein Pigment Alveograph W Area 2011 Release (%) (ppm) ( J × 10–4)

1969 1984 1990 1994 1996 1997 1998 2001 2003 2004 2008

13.5 13.3 13.2 13.2 13.4 14.0 13.2 13.2 14.1 13.5 14.2

7.5 7.8 8.6 8.4 8.9 9.0 10.1 9.9 9.3 10.4 10.0

109 109 100 200 180 110 220 175 210 310 195

0 6 0 0 0 13 8 1 66 1 5

Breeding Methodology Conventional The Swift Current breeding program follows a modified-­ pedigree method with early-­generation yield testing developed

by Hurd (1974). An off-­season (e.g., winter in the Northern Hemisphere) nursery is used to reduce the time to produce homozygous lines (Clarke et al 1998a). Approximately 50 crosses are made each year in a growth cabinet among parents that have been carefully evaluated for agronomic, disease resistance, and quality traits. From 3,000 to 10,000 plants of each cross are grown in a leaf and stem rust epiphytotic nursery in the F2 generation, and a single spike from each of the 350–400 selected plants is grown as a head row in the winter nursery near Lincoln, NZ. Typically the best 50% of the rows, showing good straw strength and maturity, are harvested. Yield testing begins in the F4 generation at two or three locations in unreplicated trials. Five spikes are selected from each line before harvest. Yield and other agronomic and disease data for each line are analyzed. Lines selected for agronomic merit are evaluated for grain protein and yellow pigment concentration by NIR spectroscopy (McCaig et al 1992).

Following selection for protein and pigment, the five spikes from selected lines are planted individually in the winter nursery.

Gluten index, kernel weight, and test weight are assessed during the winter on the grain harvested from yield trials, and lines that meet selection criteria are returned from the winter nursery for F6 yield trials. This process is repeated through the F8 generation for any lines showing visual segregation, while lines not showing segregation are entered into preregistration trials.

The breeding procedure of the University of Saskatchewan program utilizes single-­plant selection through the F4 generation (Knott 1995). The F1 and F3 are multiplied in a New Zealand winter nursery, and the F2 and F4 are grown in a rust nursery at Saskatoon. Three spikes are harvested from selected F4 plants and bulked to sow single F5 observation rows. Pigment, grain protein, and SDS-­sedimentation volume are measured on selected rows, and these are entered into yield trials in F6.

Lines from the two breeding programs that show agronomic, quality, and disease-­resistance merit are entered into replicated trials at four to six locations in the F7 or F9 generation. Lines are culled as necessary on the basis of agronomic performance and more-­detailed quality and disease assessment. The quality evaluations are pigment content, protein concentration, gluten index, semolina yield, semolina ash, and dough strength as assessed by the alveograph. Selected lines pass into a six-­location trial the following year, with the same quality determinations.

The best lines are then entered into the official registration trial, the Durum Cooperative Test, which usually comprises 30 entries grown at 11 or 12 locations. Three years of testing is required to register a new cultivar. Each year, lines are culled on the basis of agronomic performance, disease resistance, and end-­use quality relative to the major registered cultivars used as checks. After three years of testing, lines equal to or better than the checks are proposed for registration, and these proposals are voted on by committees of experts in agronomy, pathology, and cereal chemistry. Lines that are approved by the committees can then be registered by the Canadian Food Inspection Agency for

Genetics and Breeding release for commercial production. It usually takes at least 10 years to develop and register a new cultivar. Multiplication of seed for sale to producers takes a further two to three years.

The University of North Dakota breeding program also uses a modified pedigree system (Elias and Manthey 2005), making approximately 250 crosses per year, of which about 215 are planted as F2 populations of 1,000–2,000 individuals. About 125 spikes are selected from each population, and after inspection of seed, 75 are grown in F3 rows the following season, or a small portion of the populations are grown in a winter nursery near Christchurch, NZ. The F3 rows are selected for agronomic traits, disease resistance, and gluten strength, and two spikes are advanced into F3:4 rows in North Dakota or New Zealand. This is repeated for the F4:5 with the difference that no further selection is made within rows. Preliminary yield testing is performed in the F6 generation, and surviving selections are tested in more detail in the F7 and F8. More detailed quality testing, such as semolina yield and color, begins with the F6 trials. Surviving F9 lines are entered into registration trials grown at 15 locations. After three years in this trial, lines may be proposed for release, followed by seed multiplication for eventual commercial production.

In Italy, the PSB breeding program generally uses the pedigree breeding method. Each year about 50 crosses are made out of season in a greenhouse between parents having complementary positive agronomic, resistance, and quality traits that potentially can be recombined in new cultivars. F2 progenies of about 2,000 plants each are grown and artificially inoculated with a mixture of leaf rust and powdery mildew pathotypes having complementary virulence patterns. Plants with good agronomic traits (height, earliness, etc.) and more resistance than check cultivars are selected. F3 seeds produced by the selected plants are visually examined to discard plants with a high percentage of shrunken seed, black point, and yellow berry.

Yellow pigment concentration has a relatively high heritability, so selection starts at this point. The remaining seed of the selected plants is used to sow F3 rows, and the whole process is repeated. F4 rows deriving from the selected plants are sown, and fairly uniform lines are selected. Seed produced by these lines is visually examined, and test weight is measured. Protein and yellow pigment concentrations are analyzed on whole-­meal samples using NIR spectroscopy. The same samples are used for determination of SDS sedimentation value, using a microtest.

Selected F5 lines are sown in unreplicated plots for a preliminary assessment of grain yield potential, and quality is evaluated as in the previous generation.

From the F6 to the F9 generation, replicated yield trials are made in different locations. Agronomic performance and degree of disease resistance are recorded and used with yield analysis to choose the best lines. The quality of seed samples produced by these lines is evaluated in the quality laboratory of the pasta company Barilla. After test weight measurement, the wheat is ground to obtain semolina, and concentrations of protein, yellow pigment, and ash are determined. The glutograph is used to assess the gluten quality of dough. Lines remaining after at least three years of this testing are grown on larger plots, producing sufficient grain to produce a small amount of pasta in a pilot-­ scale pasta press. This pasta is similar to that obtained in com-

x

25

mercial processes and allows the final evaluation of important characteristics.

The selected lines are submitted to the registration process, which verifies that the candidate cultivars are distinct, uniform, and stable and assesses their value for cultivation and use. To be accepted for registration, the candidate cultivars must reach a threshold of yield and “quality global index,” taking into account test weight, the protein content of the semolina, gluten index, and yellow index, calculated according to the performance of a panel of the most widely grown cultivars.

Information on the breeding programs of the University of Bari and the Experimental Institute for Cereal Research, Section of Foggia can be found in Di Fonzo et al (2005).

The Australian Durum Wheat Improvement Program, Tamworth Agricultural Institute, uses a modified pedigree breeding procedure involving early-­ generation selection of highly heritable traits. Potential parents are carefully selected based on expression of agronomic, disease resistance, and quality traits. All potential parents are genetically profiled using many diversity-­array technology markers evenly distributed across the tetraploid genome. The genetic distance is calculated between pairs of potential parent genotypes, and crosses are structured to utilize parents that have a large genetic distance and are derived from distinctly different parental backgrounds. This crossing protocol is designed to at least maintain, if not enlarge, genetic variability in the breeding program. Some 80 crosses are made each year in a field nursery. F1 plants are either grown out in the glasshouse over summer or in the field during winter. F2 plants (~3,000 per cross) are space-­planted on irrigation beds.

Selected individual F2 plants are threshed as single plants (~100 per cross). Individual F2 plant progenies are tested for reaction to stripe rust, leaf rust, and stem rust at the seedling and adult plant stages and for yellow pigment levels. Selected F3 lines are field-­tested in unreplicated trials at two sites within a matrix of control cultivars. Following harvest, the F3 lines are measured for grain yield, yellow pigment, protein content, mixograph rheology characteristics, and browning reaction. Selected F4 lines are grown in yield trials at four sites. Harvested grain receives the same set of quality tests as was used for F3 lines.

The F5 to F7 lines are evaluated for yield at 16 sites across Australia, from central and southern Queensland to northern and southern New South Wales, South Australia, and Western Australia. A comprehensive set of quality tests is conducted on selected sites from each region; they include grain size, semolina yield, ash and granularity, mixograph rheology, gluten index, semolina yellow color, and (on selected cultivars) pasta traits such as texture (firmness, resilience, and overcooking tolerance), cooking loss, water absorption, and optimum cooking time. Data by region, accumulated over two to three years, are submitted for evaluation by an independent review panel before a release decision. To be acceptable for release, lines must meet minimum standards for agronomic, disease resistance, and quality criteria. Such standards are set in consultation with domestic and overseas industry partners. Pre-­and postrelease lines and cultivars are evaluated in a set of National Variety Trials that provide growers with field performance data independent of the breeders’ information. All new releases receive Plant Breeders

26

x

Durum Wheat: Chemistry and Technology, 2nd ed.

Rights registration, which allows for seed and end-­point royalty collections.

Doubled Haploids Doubled haploids created from haploid wheat plants developed by anther culture or fertilization with maize pollen are useful for genetic research and breeding. Widespread production of doubled-­haploid common wheat by the wheat × maize system became possible some years ago (Howes et al 1998, Snape 1998). However, efficiency of production of doubled haploids in durum has, until recently, been very low in comparison with that of common wheat.

Maize-­pollen-­derived haploids have been more successful than anther culture in durum (Cherkaoui et al 2000). The rate of doubled-­haploid plant production in the durum × maize system has been improved (Knox et al 2000), bringing the system close to practical usage. The hormone treatment was altered to use dicamba rather than 2,4-­D, which gave a twofold increase in the number of doubled-­haploid plants per emasculated spike.

However, this rate is still about half that achieved for the maize-­ pollen system with common wheat. There is also some effect of the durum genotype on success rate (David et al 1999, Knox et al 2000).

Both Canadian durum breeding programs and the North Dakota State program (Elias and Manthey 2005) utilize the maize-­pollen haploid technique. Cost still prohibits routine use of doubled haploids, but the cost is acceptable to permit application to genetic studies and to crosses involving complex traits or traits that are difficult to measure. Foroughi-­Wehr and Wenzel (1993) noted that doubled haploids offer a time advantage for incorporation of quantitative traits that cannot be readily selected in the early segregating generations arising from conventional crosses. The combination of doubled haploids and marker-­ assisted selection can be a very useful tool for efficient incorporation of complex traits (Howes et al 1998).

Genetic Diversity and New Traits Combating new disease or insect problems or the desire to make improvements in end-­use quality attributes requires discovery of relevant traits within durum or incorporation of those traits from other related species. Genetic transformation is another way to address such goals, but the technique is generally not accepted by consumers at this point. Wide crosses have been used to introgress genes for resistance to diseases or control of quality traits into durum from wild relatives. Such work often requires several cycles of crossing and selection due to disruption of both agronomic performance and end-­use quality attributes.

The narrow genetic base of durum is also a concern. Durum breeders have observed that crosses among closely related parents, especially backcrossing, tend to reduce the chances of selecting high-­y ielding progeny. It is therefore desirable to increase genetic diversity where possible, striking a balance between diversity and excessive disruption of economically desirable traits.

Selection for a narrow range of quality traits within a relatively small gene pool has had an impact on the current genetic diversity in cultivated durum. The fixation of major genes via the choice

of a limited number of favorable individuals has reduced the effective size of populations and consequently the genetic diversity of the whole genome. Investigations with microsatellite markers have demonstrated that modern breeding has reduced the genetic diversity available in the elite gene pool by a factor of two compared with that in landraces within the past 50 years (David et al 2003, Thuillet et al 2005). Genetic diversity in the elite gene pool represents only 16% of that available in the wild tetraploid wheat T. diccocoides L. Similar conclusions based on gene polymorphism sequences have been reported by Haudry et al (2007).

Sears (1972) was the first to suggest the use of “chromosome engineering,” which brings about wheat-­alien homoeologous pairing and recombination through the use of mutants for the main homoeologous-­pairing suppressor gene Ph1. One of such mutants, ph1c, was isolated by Giorgi in durum wheat (Giorgi 1983). Using the approach of chromosome engineering and powerful analytical tools to detect and characterize transferred chromosomal segments, Ceoloni et al (2000) produced durum wheat lines including the powdery mildew resistance gene (Pm13) from Aegilops longissima and the leaf rust resistance (Lr19) and yellow pigment (Y) genes from Agropyron elongatum. Zhang et al (2005) used a similar approach to obtain common and durum wheat carrying Lr19 and Y from Lophopyrum ponticum. Interspecific crosses followed by backcrosses are frequently used to transfer interesting traits from common to durum wheat. In this way, waxy and high-­amylose durum lines were obtained (Lafiandra et al 2006).

Wild emmer wheat (T. diccocoides L.) has been used as a source of improved protein concentration in durum (Levy and Feldman 1989). Blanco et al (2002) have researched DNA markers to facilitate such efforts. The most successful introgression of improved protein from emmer to durum has been through a chromosome substitution at North Dakota State University (Joppa and Cantrell 1990). The substitution line with T. dicoccoides chromosome 6B, Langdon (DIC-­6B), gave the most consistent increase in protein concentration, averaging 1% over four experiments. Langdon (DIC-­6B) also appeared to have the best end-­use quality of the substitution lines (Joppa et al 1991).

In subsequent studies, Steiger et al (1996) found that crosses of Langdon (DIC-­6B) with the durum cultivar Vic had a heritability of 0.92 for grain protein concentration. The authors concluded that several crossing cycles would be required to move the high-­protein genes into agronomically superior cultivars.

They attributed this to the poor agronomic nature of Langdon, which has very low yield and weak straw, rather than to genes on the substituted 6B chromosome. Research at Swift Current concluded that Langdon (DIC-­6B) did not provide yield or protein genes superior to those already available in Canadian durum germplasm (DePauw et al 1998). Markers developed by Khan et al (2000) have been used to transfer the high-­protein trait into bread-­wheat cultivars (DePauw et al 2005).

Wide crosses have also been used to increase the gluten strength of durum. Ciaffi et al (1995), for example, produced lines from a four-­way cross involving two durums, a tetraploid line from a T. turgidum × T. aestivum cross, and a T. dicoccoides parent that showed higher SDS sedimentation and alveograph work input than the durum parents. The highest gluten strength was associated with combinations of durum LMW-­2 at the Glu-­

Genetics and Breeding B3 locus and both Ax and Ay subunits coded by Glu-­A1. H. Singh et al (1998) transferred Ax and Ay subunits from T. boeticum, T. urartu, T. dicoccoides, and T. araraticum into a durum parent.

The progeny from all sources showed higher SDS sedimentation values (with the greatest increase being nearly double the SDS sedimentation value of the durum parent) in a line in which both the Ax and Ay subunits coded by Glu-­A1 were transferred from T. dicoccoides. Martin and Alvarez (2000) reported that crossing with Tritordeum, created by hyridization of durum and barley (Hordeum chilense Roem. et Schult.), would be a useful way to increase the quality diversity of durum and bread wheat, particularly in terms of creating new storage protein subunits. Alvarez et al (1999) suggested that Tritordeum could also be used to transfer new genes for high yellow pigment concentration from H. chilense into durum. A survey of 33 accessions of tetraploids representing a range of subspecies grown over two seasons were evaluated for technological quality, protein, and starch properties (Sissons and Hare 2002, Sissons and Batey 2003). A wide range in attributes was found, with some accessions useful for introgression into breeding lines to improve protein content and semolina yield.

As noted above and discussed in more detail in Chapter 4, various attempts have been made to transfer FHB resistance into durum by wide crossing because no good source of resistance to this disease has been found within durum. Much of this work has focused on moving resistance from common wheat sources such as Sumai 3 (Elias and Manthey 2005), T. dicoccoides (Stack et al 2002), T. carthlicum (Somers et al 2006), and wheatgrass (Lophopyrum elongatum; Jauhar and Peterson 2008). T. dicoccoides is also a potential source of resistance to other diseases, such as stripe rust (Uauy et al 2005).

Induction of mutations is another means to create new genetic variability; it has been used successfully in durum in the past, as in the case of the semidwarf growth habit noted above. A new nontransgenic method for reverse genetics called TILLING has been developed to induce and identify novel genetic variation. The method takes advantage of DNA sequence information to investigate the function of specific genes, but it is also used to identify mutations in a chosen gene (McCallum et al 2000, Comai and Henikoff 2006). These new mutations increase the variability available to breeders for improving specific traits. An interesting evolution of TILLING is its adaptation to the discovery of polymorphism in natural populations, called Ecotilling (Comai et al 2004).

In France, the INRA group is using diversity sampling and pre-­breeding to enlarge the genetic diversity of the durum elite gene pool. High-­throughput methods of phenotyping are used to evaluate lines generated from populations derived from crosses of elite and primitive or wild forms. Based on molecular analysis, a core collection was developed (David et al 2003).

Segregating populations have been developed by INRA and distributed to private companies to move this new diversity into breeding programs.

Marker-­A ssisted Selection Morphological, biochemical, or DNA markers can be effective tools with which to select for economic traits. For example,

x

27

Knapp (1998) showed that phenotypic selection could require screening of up to 16 times more progeny than in the case of marker-­ assisted selection, to ensure fixing a low-­ heritability trait. If a strong genetic linkage exists between the marker and the trait, the marker can be used to indirectly select for the trait.

Two further criteria must be met: it must be more efficient to screen for the marker than directly for the trait, and the parents of the population must be polymorphic for the marker. That is, one parent has the marker and the other one does not. A marker is ineffective when it does not explain a sufficient portion of the variability for the trait.

Marker-­assisted selection requires research to discover markers, to validate them, and to incorporate them into routine use (Young 1999). The discovery of a DNA marker linked to a trait is the first step (Van Sanford et al 2001), a process that requires careful characterization of a genetic population for the traits of interest. Many markers have been reported, but few are actually in routine use because considerable research effort is needed to bridge the gap between marker discovery and application (Knox and Clarke 2008). However, where all of the necessary evaluations and validations have been made, marker-­assisted selection can be a useful, practical tool in wheat breeding (Kuchel et al 2007).

Numerous DNA markers have been reported in durum, most associated with quality traits. A few examples are presented here. Elouafi et al (2001) identified three QTL that explained 62% of the genetic variation for grain yellow pigment concentration.

More recently, other authors have reported pigment QTL on other chromosomes (Pozniak et al 2007, Patil et al 2008, Zhang and Dubcovsky 2008). Blanco et al (2002) reported seven possible QTL, and Gonzalez-­Hernandez et al (2004) found three QTL, affecting grain protein concentration in durum. Suprayogi et al (2009) identified a QTL on chromosome 7A of Strongfield that contributes to an increased protein concentration of 0.4– 1.0 percentage points. Elouafi and Nachit (2004) identified two major QTL for test weight in a durum × T. dicoccoides cross, as well as five QTL for kernel weight. Recently Carrera et al (2007) identified a QTL on chromosome 4B that is linked to a deletion at the Lpx-­B1 locus, which was associated with a 4.5-­fold reduction in lipoxygenase activity and improved pasta color.

Canadian researchers have identified QTL for multiple semolina and pasta quality attributes in the mapping population W9262-­260D3/Kofa (F. R. Clarke et al 2008). Some of the QTL identified varied among environments, while others were consistent across environments. A similar pattern is shown in another mapping study with the population UC1113/Kofa (Zhang et al 2008). Some consistencies in QTL were found between the two populations, as well as some differences. This reinforces the need to validate putative markers in different populations and environments.

Disease resistance is a good target for marker development because resistance to particular pathotypes of many diseases tends to be simply inherited (Michelmore 1995). In bread wheat, markers have been developed to particular races of leaf rusts, such as Lr21 (Huang and Gill 2001) and Lr34 (Bossolini et al 2006). These markers and others permit pyramiding of resistance to multiple pathotypes of leaf rust into new cultivars, providing more durable resistance than single-­resistance genes,

28

x

Durum Wheat: Chemistry and Technology, 2nd ed.

which can be overcome by a single mutation in the pathogen.

The genetics of disease resistance has not been studied as extensively in durum, but a few efforts are under way to develop markers for leaf rust (Herrera-­Foessel et al 2007, Maccaferri et al 2008a) and leaf spots (P. K. Singh et al 2008).

Several groups have begun to look for markers for grain yield of wheat (Gonzalez-­Hernandez et al 2004, Kirigwi et al 2007, Maccaferri et al 2008b). However, development of markers for genetically complex traits such as yield is difficult because of genotype-­environmental interactions. Mathews et al (2008) investigated yield QTL over six drought-­stressed environments and assessed genotype × QTL interactions. They were able to identify only one QTL related directly to yield that was consistent across the six environments. Other QTL were significant in only a subset of environments or were colocated with other traits such as plant height and anthesis date.

Recently, a new method for rapid profiling of genomic DNA has come into use. Diversity-­array technology (DArT) can be used to simultaneously screen thousands of DNA fragments in a microarray system. The company Triticarte Pty Ltd., a subsidiary of Diversity Arrays Technology Pty Ltd. of Australia, performs DArT analysis on wheat DNA samples submitted by clients. Initially, DArT markers were not linked to a physical chromosome map, but recent efforts have begun the development of physical maps in durum (Mantovani et al 2008). This will greatly improve the utility of the technology in development of markers useful in selection.

A high-­density DArT marker map of durum wheat is being developed in Australia in association with ICARDA. This map involves some 800 DArT markers randomly spread over the tetraploid genome. Such a map will provide ready access to markers for genes and/or chromosome segments involved in both qualitative and quantitative traits of breeding interest. DArT genetic-­diversity profiles are now available for the calculation of genetic distance between pairs of prospective crossing parents.

Pairs of parents displaying a large genetic distance and derived from distinctly different parental backgrounds are selected for breeding hybrids. Meaningful genetic improvement of economically important traits is completely dependant on the availability of adequate genetic diversity in breeding populations. In the Australian program, preliminary evidence suggests that the first cycle of crossing based on genetic profiling has provided segregating lines with elevated grain protein and lutein pigment levels above those of the high-­level controls such as EGA Bellaroi.

The transfer of Nax genes from the durum wheat cultivar AUS 17045 into Australian durum cultivars has been assisted by molecular markers. Nax1 was mapped to chromosome 2AL. A very tightly linked marker, gwm312, is being used routinely to select low-­Na+ progeny in the durum breeding program (Lindsay et al 2004). Huang et al (2006) concluded that both TaHKT7-­A1 and TaHKT7-­A2 were strong candidate genes for Nax1. Nax2 has recently been mapped to chromosome 5AL. Tightly linked or perfect markers are being used for selection of lines containing Nax2 (Byrt et al 2007). The work of Byrt et al supports the hypothesis that TmHKT1;5-­A, a Na+ transporter gene, is a candidate for Nax2 and is a homoeolog of Kna1, in hexaploid wheat.

Several markers are in routine use in Canada, and more are in the process of development and validation. Selection for

low cadmium uptake was facilitated by the development of a random amplified polymorphic DNA (RAPD) marker linked in coupling to the high-­cadmium allele (Penner et al 1995). A marker generated from the RAPD primer was found to be 4.8 cM from the cadmium uptake gene and is routinely used in breeding. The marker is cheaper to use than traditional chemical analysis of cadmium. Recent work (Wiebe et al 2010) has identified markers more tightly linked to the low-­cadmium allele. Another group of markers is being used to select for resistance to the wheat midge, and a marker is being used for selection for low lipoxygenase activity, thereby reducing yellow pigment loss during pasta manufacture. Other markers are being validated for use in the Canadian durum breeding programs for traits including pigment concentration, protein concentration (Suprayogi et al 2009), gluten strength, and FHB resistance. Markers under development include those for preharvest sprouting resistance, stem and leaf rust resistance, test weight, and milling properties.

In the United States, marker-­assisted selection is being used for FHB resistance and grain protein concentration in the North Dakota breeding program. Marker development is under way for preharvest sprouting resistance (Gelin et al 2006) and leaf spot diseases (P. K. Singh et al 2008). The Canadian cadmium marker is being utilized for selection for low cadmium in the Montana State University durum breeding program.

Most of the QTL detection work has been done with biparental mapping populations. This approach has several drawbacks. It is costly to produce and evaluate a mapping population made up of an acceptable number of recombinant inbred lines. Polymorphism between parents may be limited, which has a negative impact on the number of QTL that can be discovered, and the number of alleles investigated cannot exceed the number of parents (two). Further, the QTL identified are often specific to the population analyzed.

Genome-­ w ide association mapping has been used with germplasm collections of suitable size to overcome these problems. Association mapping links genotypic (marker) and phenotypic (trait) data in a population of lines of varying degrees of relatedness (e.g., Rafalski 2002) rather than associating genotype and phenotype within a biparental cross. It offers the possibility of using existing data from breeding programs and germplasm collections rather than special crosses and may offer greater polymorphism than biparental crosses (Somers et al 2007). Research groups have collected genotypic information on groups of durum lines (Maccaferri et al 2006, Somers et al 2007) that can then be used for association mapping by collecting detailed phenotypic information on traits of interest (Sanguineti et al 2007, Reimer et al 2009).

Genetic Progress Grain Yield Durum grain yields have increased as a result of the combination of genetic and crop management improvement. Changes in crop management include appropriate nutrient application, better seeding equipment, effective control of competition from weeds, and conservation tillage systems to increase crop water use efficiency. Studies suggest that the improvements resulting

Genetics and Breeding from genetics and management are about equal (McCaig and Clarke 1995).

The rate of genetic improvement in grain yield since the release of the first Canadian cultivar, Stewart 63, in 1963, has been 0.9% per year (Fig. 2.1). Farm yield statistics show a rate of gain of about 1.6% per year, so if one assumes that 50% is genetic, it appears that the genetic gain measured in research trials is being achieved on commercial farms. The 0.9% per year gain is less than other reports, such as 4% per year since 1948 for common wheat in the United Kingdom (Austin 1999) and somewhat higher rates in some semiarid areas that started from a low yield base (Trethowan et al 2002). However, the Canadian rate of gain is similar to that reported in Spain (Garcia del Moral et al 2005) and Italy (Pecetti and Annicchiarico 1998, De Vita et al 2007).

The lower rate of genetic gain for yield in Canada compared to some other areas is probably partly attributable to the requirement to select for high protein. The grain protein concentration of Canadian durum cultivars has remained constant over time (Table 2.2), whereas concentration has decreased with increased grain yield in other areas (De Vita 2007). Data from two unselected doubled-­haploid populations suggest that a 0.6 percentage point difference in protein, such as for AC Morse vs.

AC Avonlea or Strongfield (Table 2.2), would impose an 8–15% loss in yield potential (Clarke et al 2010). However, concomitant selection for yield and protein either held yield at a similar level (AC Avonlea) or increased it (Strongfield ). A protein difference of this magnitude can be generated by about 24 kg ha–1 more available N in the Canadian environment (Selles et al 2006).

Italian durum production has fluctuated between 3.6 and 5.5 million tons per year since 1999 (Table 2.3) as a result of stress, caused mostly by drought and heat during the growing period but also sometimes by cold stress during flowering. The average yield is between 2.2 and 3.2 t ha–1. In France, durum yield increased from an average of 2.7 t ha–1 in 1970 to 5.0 t ha–1 in 2008, a rate of 0.06 t ha–1 per year.

Several studies have investigated the factors that have contributed to genetic gains in grain yield potential, with the objec-

x

tive of guiding future breeding efforts. Grain yield improvement in Italy was associated with reduced plant height, partitioning more plant resources into grain rather than into straw biomass, earlier heading, and longer duration of the grain-­fi lling growth phase (Pecetti and Annicchiarico 1998, Giunta et al 2007). De Vita et al (2007) and Giunta et al (2007) also showed that the increased number of kernels per square meter achieved by an increased number of spikes per square meter was associated with the grain-­y ield increases of Italian cultivars. Comparison of old and new Canadian durum cultivars also indicated that grain-­y ield gains were associated with shorter plant height and increased number of kernels per square meter (Wang et al 2009).

Giunta et al (2007) and De Vita et al (2007) found that modern cultivars are more responsive than older ones to nitrogen fertilizers, outyielding them at low and high levels of N input.

End-­Use Quality The cultivar Hercules was for many years the quality standard for Canadian durum. New cultivars generally maintained grain protein concentration similar to that of Hercules, while increasing yield potential relative to that of Hercules (Table 2.2). The cultivars AC Avonlea, Strongfield, and CDC Verona (Pozniak et al 2009) have substantially increased grain protein concentration relative to that in Hercules. Strongfield and other lines derived from crosses with AC Avonlea show both high protein and high yield potential (Clarke et al 2003). Grain pigment concentration and gluten strength have been increased in response to market demand (Table 2.2). Trends in quality traits are further documented in Clarke et al (2010). Adoption of new cultivars by producers has increased the average pigment content and gluten strength of the Canadian durum crop. A study of 14 Italian cultivars also reported higher gluten strength in recent cultivars but no clear trend in pigment concentration (De Vita et al 2007).

Two quality types are now found in Canada, the largest having intermediate gluten strength ranging from that of AC Morse to that of AC Navigator and the second a strong gluten type with a strength target similar to that of Commander. The market share of Strongfield reached 60% of seeded area in 2010 (Table 2.2), its fifth year of commercial production. Three new durum cultivars were registered in Canada in 2008. These are CDC Verona (Pozniak et al 2009), a line with intermediate gluten

TABLE 2.3 Durum Wheat Cultivation in Italy from 1999 to 2007a

Fig. 2.1. Genetic improvement in grain yield of durum wheat cultivars grown in Canada, expressed as a percentage of the yield of the cultivar Hercules. p = Introduced cultivars, 1 = Canadian-developed cultivars. (Reprinted from Clarke et al 2010)

29

Year

Total Area (ha)

Yield (t ha–1)

Harvest (t)

1999 2000 2001 2002 2003 2004 2005 2006 2007

1,690,633 1,663,116 1,664,195 1,733,261 1,688,834 1,772,132 1,520,061 1,342,897 1,436,758

2.77 2.69 2.23 2.58 2.26 3.20 3.00 3.05 2.79

4,514,494 4,310,331 3,624,042 4,267,831 3,717,499 5,545,706 4,431,049 3,988,736 3,911,550

a Source:

ISTAT (no date).

30

x

Durum Wheat: Chemistry and Technology, 2nd ed.

strength from the Crop Development Centre of the University of Saskatchewan, and two strong-­gluten lines, Eurostar (Clarke et al 2009a) and Brigade (Clarke et al 2009b), from Agriculture and Agri-­Food Canada. Enterprise (A. K. Singh et al 2010) was registered in 2009.

According to the “Barilla quality map,” which relies on the analysis of hundreds of wheat samples delivered to Italian mills every year, the average gluten quality and yellow index in 1997 were estimated to be 5.86 and 21.40, respectively, compared to 6.06 and 22.24, respectively, in 2007. The increase has been very small because the amount of wheat produced by modern cultivars such as Svevo and Normanno, especially selected to suit the requests of the pasta-­making industry, is relatively small compared to the total amount of durum wheat produced every year.

On the other hand, the low molecular weight subunit LMW-­1 and the high molecular weight subunit 20, known to induce poor pasta-­making quality, were more frequent in older than in modern cultivars (De Vita et al 2007).

Future prospects The challenges facing durum breeding teams are likely to be greater in the future than they have been in the past. Changes in races of pathogens and insects, changes in end-­ product processing technology, and changes in consumer tastes and expectations of food quality and safety will all have to be addressed. Global climate change is likely to affect the distribution of pathogens and insects such that new problems develop in durum-­production areas. The interaction of crops with diseases and insects is dynamic and can be unpredictable, so relatively small changes in climate could have unforeseen effects on pest severity. More variable precipitation may also affect durum production. New production areas, including soil types to which traditional durum cultivars are ill-­adapted, with qualities such as salinity, acidity, alkalinity, and micronutrient toxicity and deficiencies, will have to be utilized to meet increased demand and loss of traditionally cultivated land through urbanization and climate change. All of these problems must be addressed in addition to steadily increasing durum production to keep pace with the demands of global population growth.

Significant unexploited genetic diversity still remains in durum, as well as in relatives of durum that can be utilized in the development of new cultivars to address these challenges. New genetic tools coming into practical application will facilitate the breeding process. Transformation technology may be one of these tools if it becomes generally acceptable to producers and consumers. However, the unpredictable nature of transformation products, for example, through the up or down regulation of genes unrelated to the target transformed trait, could have undesirable consequences in relation to factors such as food safety.

The widespread and frequent cultivation of cultivars carrying herbicide-­resistance transformations has and will lead to increased selection pressure for herbicide resistance in target weed species, glyphosate resistance being a case in point. So far, very few strong candidate genes or traits have been identified for use in transformation of durum.

Innovative changes in technology continue to increase the efficiency, accuracy, and timeliness and to reduce the cost of mo-

lecular genetics research and genome sequencing. The list of species for which the complete genome has been sequenced continues to grow. This has led some molecular geneticists and science funding agencies to downplay the future importance of “traditional” crop breeding. However, this view does not recognize the reality that a genome sequence is but the first step in a hugely complex task of matching genes to DNA sequences. Proteomics has demonstrated that even if a gene (i.e., DNA sequence) is present, that gene may not be expressed in the individual all the time, or that other genes involved in regulation need to be present and active for functional gene expression. Much research is needed to understand the genetic expression and control of economically significant traits, and many gene markers still require extensive validation in practical breeding situations.

In the short term, rapid and cheap whole-­genome DNA profiling will greatly assist breeders in selection of parents that will aid in development of breeding populations with improved genetic diversity, so essential for the improvement of quantitative traits in particular. Marker-­assisted selection could be totally integrated with genome-­profiling activities. Such markers would confirm that appropriate genes or gene combinations were being included in breeding-­population development.

However, moving forward with full application of genomic information in durum breeding will require the extensive collection of phenotypic data for all important traits. In the case of quantitatively (complexly) inherited traits such as grain yield, which tend to be influenced by environment, it is difficult to obtain sufficiently detailed phenotypic data to form a clear genetic understanding of the trait. Joppa and Williams (1988) made a similar observation 20 years ago in the first edition of this monograph. Crop breeding will therefore continue to rely on extensive field evaluation to select superior cultivars.

References

Alvarez, J. B., Martin, L. M., and Martin, A. 1999. Genetic variation for carotenoid pigment content in the amphiploid Hordeum chilense × Triticum turgidum conv. durum. Plant Breed. 118:187-­189.

Araus, J. L., Amaro, T., Casadesus, J., Asbati, A., and Nachit, M. M. 1998. Relationship between ash content, carbon isotope discrimination and yield in durum wheat. Aust. J. Plant Physiol. 25:835-­842.

Austin, R. B. 1999. Yield of wheat in the United Kingdom: Recent advances and prospects. Crop Sci. 39:1604-­1610.

Ban, T., Kishii, M., Ammar, K., Murakami, J., Lewis, J., William, M., Pena, R. J., Payne, T., Singh, R., and Trethowan, R. 2005. CIMMYT’S challenges for global communications and germplasm enhancement for FHB resistance in durum and bread wheat. Pages 6-­10 in: 2005 National Fusarium Head Blight Forum. The U.S. Wheat and Barley Scab Initiative, Michigan State University Printing, East Lansing, MI. Bariana, H. S., Brown, G. N., Bansal, U. K., Miah, H., Standen, G. E., and Lu, M. 2007. Breeding triple rust resistant wheat cultivars for Australia using conventional and marker-­assisted selection technologies. Aust. J. Agric. Res. 58:576-­587.

Bhavani, S. 2006. Genetic and Molecular Mapping of Rust Resistance in Durum Wheat. Ph.D. thesis, Plant Breeding Insititue Cobbitty, Faculty of Agriculture, Food and Natural Resources, University of Sydney, Australia.

Blanco, A., Pasqualone, A., Troccoli, A., Di Fonzo, N., and Simeone, R. 2002. Detection of grain protein content QTLs across environments in tetraploid wheats. Plant Mol. Biol. 48:615-­623.