Evolution of wheat

Wheat is one of the major cereals worldwide. In 2010, the world production recorded by the International Grains Council was 679 million tonnes. Modern wheat belongs to two species, the hexaploid Triticum aestivum (2n=42 chromosomes) and the tetraploid durum type wheat T. turgidum (2n=28 chromosomes). The tribe Triticeae is economically the most important group of the family Gramineae. It has given rise to cultivated wheats, barleys, ryes, oats and an important range of grasses. The wheats include a series o f diploid, tetraploid and hexaploid forms. The polyploidy of wheat developed through amphiploidy between Triticum species and diploid species of the genus Aegilops (Nevo et al. 2002). The major grass subfamilies radiated 50-80 million years ago. Triticeae and Poaceae tribes diverged ≈35 million years ago, Hordeum and Secale diverged from the Triticum / Aegilops lineage ≈11 million years ago (Huang at al 2002, Carver 2009) and ≈7 million years ago. Progenitors of A, B, D, G and S genomes all radiated 2.5 – 4.5 million years ago(Huang at al 2002).

T. monococcum was cultivated around the 7^th millennium BC in the southern Balkans and spread to Europe (Lupton 1987). By the early Bronze Age cultivation of tetraploid (AABB) emmer wheat in and around the Fertile Crescent had enabled agricultural societies to thrive (Zohary and Hopf 1993). The cultivated tetraploid emmer wheat was probably developed from wild emmer wheat T. turgidum ssp. dicoccoides which itself is a

result of spontaneous, but rare hybridisation between T. uratu (AA) and the B genome species Ae. speltoides (Petersen et al. 2006) (Figure 8.). T. aestivum hybridization from T.

turgidum and Ae. tauschii happened only 8000 years ago (Huang at al 2002). Although repeated and independent allopolyploidisation events may have occurred, this process of speciation inevitably creates a severe evolutionary bottleneck. Modern bread wheat T.

aestivum (AABBDD) has no wild representatives and certainly arose through a further incident(s) of amphidiploidisation under the influences of human cultivation. Throughout the history of human cultivation of wheat the crop gene pool has therefore always been relatively low in genetic diversity.

A wide range of wild relatives of polyploid wheat from collections made in the Near East offer a potential germplasm source for further crop improvement by traditional breeding, and by biotechnology. The wild species occupy a wide ecological range, and a wide range of adaptive diversity to multiple diseases, pests and ecological stress (Nevo et al. 2002).

CIMMYT in particular has shown the value of resynthesizing polyploid wheat in order to introduce greater genetic diversity from representatives of the ancestral diploid species.

Most effort has been devoted to the more recent D genome progenitor Ae. tauschii. Recent strides in genetic marker technologies offer the potential for faster, targeted introgression of superior, but rare alleles, from alien species and have encouraged a renewed interest in alternative compatible sources in the A and B genomes.

Figure 8. Development of Triticum aestivum

2.5.1. Sitopsis section

Aegilops speltoides, Ae. longissima, Ae. bicornis, Ae. searsii and Ae. sharonensis species belong to the sitopsis section. The largest diversity of the genus Aegilops can be found in the Fertile Crescent from Palestine / Israel, Lebanon, Syria, Southeast Turkey, North Iraq to Northwest Iran (Kole 2011). The B genome diverged before the separation of the A and D genomes (Carver 2009). It was suggested they are the closest relatives of diploid species from where the B and G genome was derived to form the polyploid wheats (Huang et al.

2002, Furuta et al. 1976). Cytoplasmic analysis shows that Ae. speltoides was the maternal donor to polyploid wheat (Wang et al. 1997). All five species are annual diploids (2n=14), four of them are predominantly autogamous where the out crossing rate varies between 0-5 per cent and Ae. speltoides was originally allogamous with wind as the main vector. The change to autogamy could be explained by the recent decline of the population

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(Mendlinger and Zohary 1995). They are more closely related to rye than Triticum or other Aegilops sp. (Ae. tauschii and Ae. speltoides ssp. ligustica) (Huang at al 2002).

Aegilops speltoides is the most ancient species and morphologically distinct from the other species in the sitopsis section. The genetic diversity of Ae. bicornis and Ae. searsii was the lowest, while Ae. longissima and Ae. sharonensis were intermediate and Ae.speltoides was found the most polymorphic species of the group (Goryunova et al. 2008).

2.5.2. Einkorn wheat

Einkorn wheat incorporates two biological species T. monococcum and T. urartu (Nevo et al. 2002). In early cytogenetic studies in the 1920-1930s it was shown the A genome of the tetraploid species T. turgidum and T. timopheevi was derived from T. monococcum, but in the 1970s Chapman and his team believed the A genome donor was T. urartu. Further studies of the seed storage proteins indicated the A genome of T. turgidum was originated from T.urartu and in T. timopheevi, the A genome was contributed by T. monococcum.

Triticum monococcum is the only cultivated diploid wheat, which was found by Boissier in Greece in the 1880s. Recent studies showed that the variation in the A genome repeated nucleotide sequences, present in both tetraploid species, was more related to T.urartu than T. monococcum. This indicates that T. urartu is the A genome donor of all the polyploid wheat species (Carver 2009).

Triticum monococcum possesses a high level of resistance to a range of diseases and pests including: Pseudocercosporella herpotrichoides (Cadle and Murray 1997), leaf rust (Hussien et al. 1998, Bartoš et al. 2005) powdery mildew (Yao et al. 2007) stem rust (Plamenov et al. 2009), Septoria (Jing et al. 2008) and aphids (Sitobion avenae and Diuraphis noxia) (Potgieter et al. 1991, Migui and Lamb 2004). Cakmak et al. (1999) introduced genes from T. monococcum into hexaploid species to increase zinc uptake efficiency and Sodkiewicz (2002) reported similar work to prevent pre-harvest sprouting.

For further crop improvement by traditional breeding a wide range of A and B genome wild relatives exist that are cross compatible and could be used for introgression (Nevo et al. 2002).