Hexaploid Wheat: Evolution, Domestication, Dissemination, US commercialization, and Molecular characterization
Wheat is one of the major food crops in the world, which provides 532 kcal/capita/day (http://faostat.fao.org). World production of wheat is about 670 M tonnes, which is only behind maize (872 M tonnes) and rice (720 M tonnes) (http://faostat.fao.org) whereas the US is the third largest wheat producer in the world (http://faostat.fao.org). Wheat group comprise of 13 diploid and 18 allopolyploid species (12 tetra and 6 hexa) (Feldman et al. 2012), where hexaploid wheat is the most cultivated class throughout the world (Faris 2014). Hexaploid wheat (bread wheat) is known as the allohexaploid (2n=6x=42, BBAADD), because it is derived from the diploid species via convergent evolution (Faris 2014 and Feldman et al. 2012). Hexaploid wheat consist of winter and spring wheat, which mainly differed by vernalization governed by vrn (vrn1, vrn2) genes (Taiz and Zeiger 2002, Doebley et al. 2006).
Wheat has 21 pairs of homologous chromosomes and seven homoeologous chromosome group, which possess high level of similarity (Feldman et al. 2012). There are various genetic changes such as elimination of low or high copy DNA sequences, elimination of rRNA and 5S RNA genes and intergenomic invasion of DNA sequences occur at the time of allopolyploidization which vacillate the speciation process (Feldman et al. 2012; Feldman et al. 2012). The ph locus present on 5BL play an important role in the genetic interaction between genomes (Feldman et al. 2012). It lead to the formation of bivalent; due to this reason wheat chromosomes behave like diploids during meiosis (Feldman et al. 2012).
In wheat, the both progenitors of Triticum and Aegilops derived from a common ancestor having 7- chromosome around 3.0 MYA year ago (Faris 2014). Two amphiploidization events took place in the evolution of the hexaploid wheat. The first event took place between T.uratu (AA) (Br13Aq5A) (Dvorak et al. 1993) and Ae. Speltoides ssp.lingustica (SS) (Br13S Tg22Sq5S) around 0.5 MYA ago. This event led to the formation of T.turgidium ssp. dicoccoides (AABB) (Br13ABr13BTg22Bq5A q5B), which evolved to T.turgidium (br13Abr13Btg22BQ5A q5B), the free threshing tetraploid (Faris 2014). The progenitor of T.turgidium is uncertain .It either belongs to the dicoccum or parvicoccum subspecies but there are lots of factors that lead to this uncertainty, including brittle rachis, the waxiness gene (Faris 2014).
The second amphiploidization event between T.turgdium (AABB) and Ae. Tauschii (DD) led to the formation of the hexaploid wheat (Dvork et al. 1998; Lelley et al. 2000; Faris 2014) T.aestivum ssp spelta (Asian or Asian like), which has hulled seeds due to the presence of Tg1 from the Ae. Tauschii. The B, G, S genomes diverged from Ae.spletoides (SS), but still have some common features (Zhang et al. 2002; Kilian et al. 2006; Faris 2014). Further evolution of T.aestivum ssp spelta led to...