Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

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The two lineages have diverged from one another during the Miocene, about 8–15
MYA (Wolfe et al. 1989 ; Ramakrishna et al. 2002 ; Huang et al. 2002a , b ; Dvorak and
Akhunov 2005 ; Chalupska et al. 2008 ; Fan et al. 2013 ; Middleton et al. 2013 , 2014 ;
Marcussen et al. 2014 ; Gornicki et al. 2014 ; Table 2.4 ). Based on analysis of nuclear
genes, Marcussen et al. ( 2014 ) concluded that the ancestors of the wheat group have
diverged from rye during the Miocene, about 7 MYA, while, on the basis of analysis
of chloroplast DNA, Middleton et al. ( 2014 ) suggested that the divergence from rye
was in the Pliocene, 3–4 MYA. The diploid Triticum and Aegilops species started to
diverge from one another during the Pliocene, about 2 to 4 MYA (Huang et al. 2002b ;
Dvorak and Akhunov 2005 ; Middleton et al. 2014 ), and the allopolyploids of this
group were formed between 0.01 and 1.0 MYA, during the Pleistocene and Holocene
(Huang et al. 2002b ; Dvorak and Akhunov 2005 ; Marcussen et al. 2014 ; Gornicki
et al. 2014 ; Tables 2.3 , 2.4 , 2.7 , 2.9 and 2.10 ; Fig. 2.1 ).
Fan et al. ( 2013 and reference therein) found a major radiation of the Triticeae
during a relatively narrow period of time (6.1–9.2 MYA). It is therefore assumed
that the radiation of the Triticeae might have been triggered by the late Miocene
climate that was warmer in summer and cooler in winter, simultaneously with the

Table 2.3 (continued)

Period Epoch

Million

years ago

Major climate and

ecological events

Events in the

evolution of the

Triticeae

Miocene 23.8–5.3 Warmer global climates;

towards the end,

disappearance of the

Tethys Sea and the

climate cooled off;

diversifi cation of

temperate ecosystems

and new ecological

niches opened;

expansion of grasslands;

Diversifi cation of

grasses; divergence

of the Triticinae

(wheat lineage)

from the Hordeinae

( barley lineage)

Tertiary Oligocene 33.7–23.8 Cold and dry climates;

transformation of

vegetation to something

similar to that of today

Appearance of early

Triticeae

Eocene 54.8–33.7 Warm and humid

climates, became cooler

towards the end; forests

got smaller and

grasslands and savannas

increased

Further development

of grasses

Paleocene 65.0–54.8 Mild and uniform

climate; mammals

became the dominant

land-living life form;

some plant forms (pines,

cacti, palms) fi rst

appeared

Appearance of early

grasses

M. Feldman and A.A. Levy