Case Study: Wheat Domestication and Breeding
Patrick Byrne, Dept. of Soil and Crop Sciences, Colorado State University, Fort Collins, Colorado 80523<EMAIL_ADDRESS>Outline
- Introduction
- A and B genome ancestors and A x B hybridization
- D genome ancestor and AB x D hybridization
- Synthetic hexaploid wheat
- Useful genes from wild wheat relatives
- References
- Acknowledgments
1. Introduction
The domestication of wheat (Triticum aestivum L.) from wild grasses in the Middle East is a fascinating story that resulted in one of the world’s most important and widespread crops. It is estimated that wheat provides about 20% of the energy in global diets and about the same percentage of protein (WHEAT, 2014). Wheat is primarily a cool season crop, but is broadly adapted to many types of growing conditions, both irrigated and rainfed, and is especially important as a staple crop in semi-arid conditions. Part of wheat’s adaptability is due to the fact that it occurs both as winter habit varieties (requiring a cold period of 6 to 8 weeks to trigger reproduction), and spring habit varieties (not requiring the cold period). Wheat geneticists and breeders have a long history of identifying useful genes in wild wheat relatives and incorporating them into improved varieties.
2. A and B genome ancestors and A x B hybridization
Common wheat is a hexaploid species with three sets of similar but distinct chromosomes. These chromosome sets are designated the A, B, and D genomes, each with seven pairs of chromosomes. Thus, hexaploid wheat contains 3 genomes x 7 pairs = 21 pairs or 42 chromosome total. Each genome originated in a different annual diploid grass species in the Fertile Crescent of the Middle East (Figure 1).
Figure 1. The Fertile Crescent of the Middle East, where wheat and many other crops were domesticated.
Figure 2. The combination of the A, B, and D genomes led to common bread wheat, which has all three genomes. From left to right, A genome, Triticum urartu; B genome, Aegilops speltoides ligustica; D genome, Aegilops tauschii; and A+B+D genome, Triticum aestivum. Photo credit: Pat Byrne.
The A genome ancestor of wheat is Triticum urartu, and the B genome is thought to have originated with a close relative of Aegilops speltoides. Hybridization between these progenitors less than one million years ago (Marcussen et al., 2014) gave rise to the tetraploid species Triticum turgidum ssp. dicoccoides, known as wild emmer, having the AABB genome constitution. This wild species was domesticated to form emmer wheat (Triticum turgidum ssp. dicoccum), which gave rise to durum or pasta wheat (Triticum turgidum ssp. durum). Both emmer and durum wheats are tetraploids with genome designation AABB.
A separate lineage of the A genome led to domesticated einkorn wheat (Triticum monococcum ssp. monococcum), which is still grown in remote parts of Turkey, Italy, and Spain. The genome of this diploid species is usually designated AmAm to distinguish it from the lineage