Evolution and origin of bread wheat

Abstract

Bread wheat (Triticum aestivum, genome BBAADD) is a young hexaploid species formed only 8,500-9,000 years ago through hybridization between a domesticated free-threshing tetraploid progenitor, genome BBAA, and Aegilops tauschii, the diploid donor of the D subgenome. Very soon after its formation, it spread globally from its cradle in the fertile crescent into new habitats and climates, to become a staple food of humanity. This extraordinary global expansion was probably enabled by allopolyploidy that accelerated genetic novelty through the acquisition of new traits, new intergenomic interactions, and buffering of mutations, and by the attractiveness of bread wheat's large, tasty, and nutritious grain with high baking quality. New genome sequences suggest that the elusive donor of the B subgenome is a distinct (unknown or extinct) species rather than a mosaic genome. We discuss the origin of the diploid and tetraploid progenitors of bread wheat and the conflicting genetic and archaeological evidence on where it was formed and which species was its free-threshing tetraploid progenitor. Wheat experienced many environmental changes throughout its evolution, therefore, while it might adapt to current climatic changes, efforts are needed to better use and conserve the vast gene pool of wheat biodiversity on which our food security depends.

Figure 1

Phylogenetic representation of wheat evolution. Wheat evolution is shown starting ∼7 MYA from a progenitor that gave rise to the (A), (B), and (D) lineages that merged to form bread wheat. The relative timing of the major speciation events is shown in the horizontal axis and described in the boxes above. The tree is adapted from Glémin et al. (2019); Li et al. (2022); Avni et al. (2021). The diploid species related to the D genome (asterisks) are Aegilops bicornis, Ae. longissima, Ae. searsii, Ae. sharonensis, Ae. caudata, Ae. comosa, Ae. umbellulata, and Ae. uniaristata.

Figure 2

Archaeological evidence for wheat cultivation and domestication in the near-east. The location of the fertile crescent is shown as dashed green lines. Its boundaries correspond to the distribution of wild progenitors of wheat, barley, and several legumes as well as to early domestication of these crops. The western part, called the Levant or levantine corridor (Bar-Yosef, 1998), goes south, around the Jordan valley between the dashed line on the side of the Syrian desert and the Mediterranean Sea. The south Levant is the region between Beidha (#6) and Aswad (#9) and the north Levant is north of Aswad (#10), for example, in Dja’de (#5) and Abu Hureya (#6). The northern area of the fertile crescent is also referred to as the upper Euphrates (e.g. Cafer Hoyuk, #11), and the east of the fertile crescent, in the Zagros mountain is represented by sites such as Chogha Golan (#4). The years on the blue horizontal axis correspond to “Calibrated years before present” (Cal BP). The bottom boxes represent the climatic and the archeological periods when cultivation and domestication took place. Horizontal lines flanked by the location number (see map) and names indicate the relevant period when archeological evidence of cultivation or domestication was found. Numbers in red indicate two regions where evidence came from genomic data rather than archaeological data for the putative progenitors of domestic emmer (#13) and for the donor of the D subgenome of bread wheat (#14). Archaeological data were gathered from Nesbitt (2001); Willcox (2012); Zeder (2011); Riehl, et al. (2013). A blank topographic map from Wikipedia (Middle East topographic map-blank 3000bc.svg, by Fulvio314, CCBY 3.0) served as the background on which text and data were added.

Figure 3

Major mutations and morphological changes during wheat domestication. 1, The transition from ssp. dicoccoides to ssp. dicoccon involved mutation in the Brittle rachis loci. Some modern emmer wheat lines might also contain mutations in some but not all loci affecting free threshing. 2, Free-threshing tetraploid wheat, named ssp. parvicoccum, appears in the archeological record approximately 2,000 years before ssp. durum. It is now extinct but might have resembled the tetraploid wheat (Genome BBAA) shown here as ssp. X that was extracted from hexaploid wheat and has a compact spike and small grains. Its genotype must have been similar to durum, namely free threshing with soft glumes, with mutants Q and tg-A1, tg-B1. 3, The hybridization of this free-threshing tetraploid wheat with the DD subgenome donor, Ae. tauschii, gave rise to a primitive hulled hexaploid wheat, different from spelt wheat due to the Q factor, and absent from the archaeological record. It likely resembled the picture shown from a synthetic hexaploid between ssp. durum and Ae. tauschii shown here. 4, Soon after its formation, hexaploid wheat became free threshing thanks to a mutation in Tg-D1 and its rachis became thicker thanks to a mutation in Br-D2.