Patrick Byrne, Dept. of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523 (Patrick.Byrne@colostate.edu)
Above photo credits (left to right): Phil Westra, Pat Byrne, Mike May.
- 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
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.
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 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 that led to hexaploid wheat.
Wheat’s D genome ancestor is the goatgrass Aegilops tauschii, which occurs across a broad range of the Middle East and Asia from Syria to China. DNA analysis of wheat and its wild relatives led Marcussen et al. (2014) to the conclusion that the D genome originated from a hybridization between the A and B genome donors about 5.5 million years ago. Much later, another hybridization occurred between domesticated emmer wheat (AABB) and Ae. tauschii (DD), giving rise to hexaploid common or bread wheat (T. aestivum), with the AABBDD genome designation. This hybridization is believed to have occurred only 8000 to 10,000 years ago, most likely in an area south of the Caspian Sea in present-day Iran (Gepts, 2018). The D genome provided wheat with adaptability to the harsh environmental conditions of Central Asia, as well as proteins that improved the breadmaking properties of the flour.
The hybridization that led to common wheat is believed to have involved a very limited number of Ae. tauschii plants. Thus, the D genome is the least genetically diverse of wheat’s three genomes. The diversity present across the vast range of Ae. tauschii is mostly absent in today’s wheat and until recently has been largely unexplored as a source of useful variation.
A creative strategy for addressing the impoverished D genome diversity of common wheat is the development of synthetic hexaploid wheats (McFadden and Sears, 1946; Trethowan and Mujeeb-Kazi, 2008; Ogbonnaya et al., 2013). This strategy is an attempt to recreate the evolutionary history of wheat by intentionally crossing tetraploid (AABB) wheat with Ae. tauschii (DD), but using a much broader range of Ae. tauschii accessions than occurred in nature. Ae. tauschii is an attractive species for introgression into common wheat, because its genome readily pairs and recombines with wheat’s D genome chromosomes. The International Maize and Wheat Improvement Center (CIMMYT) has been a leader in producing and utilizing synthetic hexaploids, with over 1,500 synthetics developed (Mujeeb-Kazi and Hettel, 1995; Rosyara et al., 2019).
The process of developing synthetic hexaploid wheat is diagrammed in Figure 3. Because the initial product of fertilization between the tetraploid and diploid species is inviable after a few weeks, an embryo rescue procedure must be used in many cases. This involves dissecting out the young embryo and growing it in tissue culture to produce a triploid (ABD) plant. The chromosome number of this plant is then doubled by treating with colchicine to produce AABBDD plants, the same as common wheat. Other methods for incorporating wild species diversity into wheat include direct crossing of Ae. tauschii or wild tetraploid accessions to hexaploid wheat, as explained in Ogbonnaya et al. (2013).
Synthetic hexaploids are not directly useful for wheat production because they are typically not adapted to the target growing area and they express wild species traits, such as poor threshing (i.e., the seeds do not separate easily from the enclosing glume tissue) and excessive lodging (Figure 3). Therefore, the synthetic lines are typically crossed to an adapted wheat cultivar, then backcrossed once or twice to the adapted cultivar, followed by selection. This results in breeding lines (sometimes called synthetic-derived lines) that have mostly adapted genomes with segments of synthetic hexaploid chromosomes interspersed. These lines can be grown and evaluated like normal wheat in hopes of finding improvements in qualitative or quantitative traits due to the inserted synthetic hexaploid segments.
Figure 4. Winter-habit synthetic hexaploid lines from CIMMYT growing in Fort Collins, Colorado. Photo credit: Pat Byrne.
CIMMYT has done extensive evaluation of its synthetic-derived lines (Dreccer et al., 2007; Lage and Trethowan, 2008; Rosyara et al., 2019). Ogbonnaya et al., 2013 state that in the 7 years previous to their review, 17% of wheat lines in all of CIMMYT’s international trials were synthetic-derived, but that number increased to 35% for the Semiarid Wheat Yield Trial. In a study of 253 synthetic-derived lines relevant to CIMMYT’s breeding and testing program, the average marker-based estimate of the Ae. tauschii contribution to those lines was 17.4% (Rosyara et al., 2019). Over 80 synthetic-derived lines have been released as cultivars, notably Sokoll and Vorobey (Rosyara et al., 2019).
Synthetic hexaploid lines have been shown to be useful sources of variation for abiotic and biotic stress tolerance, as well as agronomic and novel grain quality traits. Their improved yield performance under drought stress conditions appears to be due at least in part to shifts in their rooting profile, whereby a greater proportion of root biomass is produced deeper in the profile (Lopes and Reynolds, 2011; Reynolds et al., 2008).
Another approach to uncovering useful variation in Ae. tauschii accessions is to develop a Nested Association Mapping population (NAM), a method described for maize by Yu et al. (2008). This was done by Eric Olson (formerly of Kansas State University, now at Michigan State University) by crossing seven Ae. tauschii accessions directly to the Kansas adapted wheat line KS05HW14, followed by two backcrosses and several generations of self-pollination. The resulting population of 420 lines (named the DNAM) is shown growing in Video 4.
Genes from Aegilops species and other wild relatives have been extremely important for improving a number of traits in wheat, especially disease resistance. Kishii (2019) has compiled genes that have been identified or transferred from Ae. tauschii to wheat. These includes genes for resistance to leaf rust, stem rust, stripe rust, powdery mildew, Septoria tritici, Septoria nodorum, tan spot, cyst nematode, root knot nematode, Hessian fly, greenbug, Russian wheat aphid, wheat curl mite, and soil-borne cereal mosaic virus. Another table in Kishii (2019) compiles genes from Aegilops species other than Ae. tauschii. This list includes genes for resistance to eyespot, leaf rust, stem rust, stripe rust, powdery mildew, cyst nematode, root knot nematode, and greenbug.
The Wheat Genetics Resource Center at Kansas State University has been a leader in collecting, conserving, and exploiting wild wheat relatives for crop improvement. Germplasm released by the Center includes breeding lines for disease and insect resistance, bread making quality, and other traits.
In the realm of abiotic stress tolerance, Placido et al. (2013) showed that a chromosome segment translocation from Agropyron elongatum provided wheat with the ability to maintain root growth under moisture stress conditions.
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Citation: Byrne P. 2020. Case Study: Wheat Domestication and Breeding. In: Volk GM, Byrne P (Eds.) Crop Wild Relatives in Genebanks. Fort Collins, Colorado: Colorado State University. Available from https://colostate.pressbooks.pub/cropwildrelatives/chapter/wheat-breeding-with-crop-wild-relatives/
This training module was made possible in part by funding from USDA-ARS, Colorado State University, IICA-PROCINORTE (procinorte.net), and the United States Agency for International Development (USAID).
Editors: Emma Balunek, Gayle Volk
Videographer: Mike May