Durum Wheat: Chemistry and Technology, Second Edition

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

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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).

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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.

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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

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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)

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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

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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.

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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)

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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

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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

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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.

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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

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