104 Polyploidy and the Domestication of Wheat

Key Idea: The evolution of wheat involved two natural hybridisation events, accompanied by polyploidy. Wheat has been cultivated for more than 9000 years and has undergone many changes during the process of its domestication including hybridisation and polyploidy. Hybrids are the offspring of genetically dissimilar parents. They are important because they recombine the genetic characteristics of parental lines and show increased heterozygosity. This is associated with greater adaptability, survival, growth, and fertility in the offspring (called hybrid vigour). There is evidence to show that interspecific hybridisation (hybridisation between different species) was an important evolutionary mechanism in the domestication of wheat. Polyploidy has also played a major role in the evolution of crop plants. Most higher organisms are diploid, i.e. have two sets of chromosomes (2N), one set derived from each parent. Diploids formed from hybridisation of different species are usually infertile because the two sets of chromosomes cannot pair properly at meiosis. In such hybrids, there are no gametes produced or the gametes are abnormal. In some cases of allopolyploidy, the chromosomes can be doubled and a tetraploid is formed from the diploid. This restores fertility to a hybrid, because each of the original chromosome sets can pair properly with each other during meiosis. These processes are outlined below.

Polyploidy events in the evolution of wheat

The wild einkorn becomes domesticated in the Middle East. Slight changes in the phenotype occur, but not in the chromosome number.

Wild einkorn

Genome

2N no. Einkorn

Genome

2N no.

The sterile hybrid undergoes amphiploidy (an allopolyploidy event involving doubling the chromosome number in a hybrid between two species). This creates the fertile emmer wheat.

These two species interbred to form a hybrid and would have initially been sterile.

X

Wild grass

Genome

2N no.

Common wheat

Genome

2N no. Goat grass

Genome

2N no.

The table on the right and the diagram above show the evolution of the common wheat. Common wheat is thought to have resulted from two sets of crossings between different species to produce hybrids. Wild einkorn (14 chromosomes, genome AA) evolved into einkorn, which crossed with a wild grass (14 chromosomes, genome BB) and gave rise to emmer wheat (28 chromosomes, genome AABB). Common wheat arose when emmer wheat was crossed with another type of grass (goat grass).

Common name

Wild einkorn

Einkorn

Wild grass Emmer wheat

Goat grass Common wheat

These two species interbred to form a hybrid and would have initially been sterile.

Emmer Wheat

Genome

X

2N no.

Sterile hybrid undergoes amphiploidy, doubling the number of chromosomes to create the fertile common wheat.

Species Genome Chromosomes N

Triticum aegilopiodes Triticum monococcum

Aegilops speltoides Triticum dicoccum

Aegilops squarrosa Triticum aestivum

AA

AA

BB

AABB

DD

AABBDD 7

7

7

14

7

21

Ancient cereal grasses had heads which shattered readily so that the seeds would be scattered widely. Modern wheat has been selected for its non-shattering heads, high yield, and high gluten content.

Teosinte

Modern corn

Corn has also evolved during its domestication. Teosinte is thought to be the ancestor to both corn and maize.

1. Using the table on the previous page, label each of the wheats and grasses in the diagram with the correct genome and 2N chromosome number for each plant.

2. Explain the term hybrid vigour:

3. Discuss the role of polyploidy and interspecific hybridisation in the evolution of wheat:

4. Cultivated wheat arose from wild, weedy ancestors through the selection of certain characters.

(a) Identify the phenotypic traits that are desirable in modern wheat varieties:

(b) Suggest how ancient farmers would have carried out a selective breeding programme:

5. Cultivated American cotton plants have a total of 52 chromosomes (2N = 52). In each cell there are 26 large chromosomes and 26 small chromosomes. Old World cotton plants have 26 chromosomes (2N = 26), all large. Wild

American cotton plants have 26 chromosomes, all small. Briefly explain how cultivated American cotton may have originated from Old World cotton and wild American cotton:

6. Why is it important to maintain the biodiversity of wild plants and ancient farm breeds?