referred to as subspecies (subsp.) in other classification systems. The cultivated forms of wheat have traits collectively known as the domestication syndrome. Two traits have been particularly important in the domestication of wheat. The first is the loss of shattering of the spike at maturity through having a semi‐tough rachis. It is important for the rachis of mature ears of wild plants to be brittle, such that the ear shatters or disarticulates to aid seed dispersal. By contrast, mutations at the Br (brittle rachis) loci on the short arms of group 3 chromosomes (Nalam et al. 2006) confer a semi‐tough rachis that keeps the ear intact. This prevents seed loss before manual harvesting; although this is highly advantageous to gatherers and farmers, it represents a significant loss of fitness for the wild plant. The appearance of the non‐shattering phenotype in the archaeological record is, therefore, often assumed to be confirmation of wheat cultivation (Allaby et al. 2017). However, it is also likely that cultivation of wild types predated that of domesticated forms. Both shattering and non‐shattering forms are found within the Fertile Crescent dated to the same time, and often within the same site (Tanno and Willcox 2012). There are very early dates for the first possible appearance of non‐shattering einkorn (> 11 000 BP) and emmer (> 10 000 BP), at camps or settlements where shattering types dominated (Allaby et al. 2017). Thereafter the shift from non‐shattering to shattering forms was a slow evolutionary process. The greatest selection pressures from the presumed adoption of cultivation are estimated to be from 10 500 to 9500 BP (Allaby et al. 2017), but the complete transition to domesticated forms probably took three to four millennia (Fuller et al. 2012). Today, all cultivated forms of wheat have a semi‐tough rachis.
The second important change in domestication has been the transition from hulled forms in which the grain is harvested within glumes and other spikelet structures (Figure 1.12), to free‐threshing forms in which grains are released naked during harvesting processes. The free‐threshing trait of wheat arose by a dominant mutant at the Q locus which modified the effects of recessive mutations at, e.g. the Tg (tenacious glume) locus on the group 2 chromosomes (Dubcovsky and Dvorak 2007; Kilian et al. 2009). Not all cultivated forms of wheat yield naked grain at harvest. For example, most cultivated einkorns are hulled ( T. monococcum var. monococcum). However, the var. sinskajae has free‐threshing grain and can be used as a source of this trait in einkorn breeding programmes.
Wild emmer ( T. turgidum var. dicoccoides) has given rise to several cultivated tetraploid variety groups. Cultivated emmer (var. dicoccum) is hulled but the most widely grown form is free‐threshing durum wheat (var. durum). Durum wheat accounts for about 5% of global wheat production. It is particularly adapted to Mediterranean climates and is mostly used for making pasta and regional foods such as couscous and bulgar. Other free‐threshing variety groups are only cultivated to a limited extent. These minor groups include rivet wheat (var. turgidum), Polish wheat (var. polonicum), Persian wheat (var. carthalicum), and Khorasan wheat (var. turanicum). T. timopheevi is represented in both hulled (var. timopheevi) and naked forms (var. militinae) and is still cultivated in small areas in the Caucasus.
Cultivated einkorn and emmer clearly developed from the domestication of natural populations. By contrast, hexaploid wheat ( T. aestivum, genome formula AABBDD) has never existed as a wild species. It originated through hybridisation of cultivated emmer with the wild goat grass Aegilops tauschii (also called Triticum tauschii and Ae. squarrosa), which contributed the D genome (Figures 1.13 and 1.15). This hybridisation probably occurred several times, with farmers selecting hexaploid lines for their larger grains and higher yields. Modern wild populations of Ae. taushcii are found growing in the north and northeast of the Fertile Crescent and populations carrying genomes closely related to the D genome of hexaploid wheat occur in Transcaucasia, between the Black Sea and the Caspian Sea (particularly in modern‐day Armenia), and around the southwest of the Caspian Sea (modern‐day northern Iran) (Dvořák et al. 1998). The first hexaploid wheats may, therefore, have occurred around the south or west of the Caspian Sea about 8000 BP (Kilian et al. 2009).
1.3.4 The Spread of Wheat Cultivation
The cultivation of bread wheat spread via Anatolia to Greece, and then both northwards through the Balkans to the Danube (7000 BP), and westwards to Italy, France, and Spain (7000 BP), reaching the UK and Scandinavia by about 5000 BP. Similarly, wheat spread via Iran into central Asia, reaching China by about 3000 BP, and to Africa, initially via Egypt. It was introduced by the Spanish to Mexico in 1529 CE and by the British to Australia in 1788 CE. These migration routes have been described in detail by Feldman (2001).
This wide geographic expansion of wheat was undoubtedly facilitated by the allohexaploidy of T. aestivum . The environmental and geographic distributions of allopolyploids are often wider than for their diploid parents (Miller 1987). Furthermore, T. aestivum has adapted to a wider range of environments than the tetraploid T. turgidum (Dubcovsky and Dvorak 2007). Feldman and Levy (2005) list several mechanisms by which allopolyploidy has led to the rapid evolution of wheat genomes and gene expression. The multiple genomes would be expected to confer fixed hybrid vigour, genetic buffering, and evolutionary adaptability (Simmonds and Smartt 1999). Having multiple copies of similar genes means that mutations in one or two copies can occur without catastrophic effects. This can, for example, allow the development of intermediate types and better fine tuning of growth and development patterns to the environment. The addition of the D genome appears to have been particularly important given the different and wider geographic distribution and environmental adaptation of Ae. taushcii compared with the wild donors of the A and B genomes. Modern wild stands of Ae. taushcii extend beyond the Mediterranean zones into continental climates (Zohary et al. 1969).
Although T. aestivum is one of the youngest major crop species, it is now also one of the most diverse. Feldman (2001) estimated that about 25 000 genotypes were available globally at the turn of the millennium, but this is clearly an underestimate as material present in countries such as China was not readily available at the time of publication. Although several genotypes of Ae. tauschii contributed to the formation of hexaploid bread wheat, most of the diversity in bread wheat has clearly arisen since the initial hydridisation occurred. Dubcovsky and Dvorak (2007) suggest that this has several origins. Firstly, extensive gene flow has occurred between bread wheat and cultivated and wild forms of tetraploid wheat, particularly wild emmer (He et al. 2019; Allen et al. 2021), capturing diversity from these species. However, gene flow into the D genome has been more limited; it remains less diverse than the A and B genomes (Allen et al. 2021). Secondly, bread wheat has an unusually large genome, comprising about 17Gb of DNA, (about 40 times the size of the rice genome). About 85% of the genome is non‐coding repetitive DNA including mobile elements, which facilitates the generation of variation by gene duplication and deletion and by insertion into regulatory and coding sequences. These events are more likely to be tolerated due to the buffering capacity of the three genomes. Hence, the combination of gene flow from related wild species, polyploidy, and high genome plasticity have contributed to the variation which has developed over the past 10 000 years.
The dominant grouping of T. aestivum is modern, free‐threshing, bread wheat (var. aestivum). This form accounts for about 95% of current global wheat production and is widely used to make bread, other baked goods including cakes and biscuits, Asian noodles (but not pasta), and as an ingredient in food processing. Two additional naked forms are club wheat (var. compactum), which has highly compact heads and is grown principally in the USA (Pacific Northwest and California), and Indian shot wheat (var. sphaerococcum), which is characterized by rounded grains and is from northwest India and Iran. T. aestivum also occurs in three hulled forms, the most widely cultivated of which is spelt (var. spelta). The other recognised hulled variety groups
