Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

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later, about 2.5–4.5 (MYA) and Dvorak and Akhunov ( 2005 ) suggested that the
divergence time of these species these was about 2.7 (1.4–4.1) MYA (Table 2.7 ). In
agreement with the conclusion of Dvorak and Akhunov ( 2005 ), recent data from
sequencing of whole chloroplast genome indicated that the species having the A, B,
and D genomes began to diverge between 2.1 and 2.9 MYA (Middleton et al. 2014 ;
Gornicki et al. 2014 ; Tables 2.4 and 2.7 ). Thus, it is reasonable to assume that the
diploid species of the group evolved at different times, the primitive species about
2.5–3.0 MYA and the advanced ones, namely, Ae caudata , Ae. comosa , Ae. unia-
ristata and Ae. umbellulata, diverged from the basal species later on, about 2.5
MYA (Marcussen et al. 2014 ). This period corresponds to the geological epoch
Pliocene (1.8–5.3 MYA; Table 2.3 ), that was characterized by development of
seasonal climate (cold and humid winters and hot and dry summers) in the east
Mediterranean and Southwest Asia, the center of origin of the diploid species of this
group. The adaptation to dry habitats with seasonal growth periods led presumably
to the development of annual growth habit associated with increased self- fertilization
and large grains.
Aegilops speltoides started to diverge from subspecies Emarginata ( Ae. bicornis,
Ae. longissima, Ae sharonensis, and Ae. searsii ) about 2.5 MYA (Gornicki et al. 2014 ).
Wang et al. ( 1997 ) and Mendlinger and Zohary ( 1995 ) found that Ae. sharonensis ,

Table 2.7 Time of beginning divergence of the diploid genome s of the wheat group (species of
the genera Aegilops and Triticum ) in million years ago

Genomes

Beginning

divergence

time

DNA sequences

studied Reference

Genomes A and S 6.5 Several hundreds

nuclear genes

Marcussen et al. ( 2014 )

2.3 Chloroplast DNA Gornicki et al. ( 2014 )

Genomes A and D 5.50 Several hundreds

nuclear genes

Marcussen et al. ( 2014 )

2.70 (1.4–4.1) Four nuclear genes Dvorak and Akhunov ( 2005 )

2.30 Nuclear genes Chalupska et al. ( 2008 )

1.20 Chloroplast DNA Gornicki et al. ( 2014 )

Genome s S and D 5.5 Several hundreds

nuclear genes

Marcussen et al. ( 2014 )

2.3 Chloroplast DNA Gornicki et al. ( 2014 )

Genomes A m and A 0.57 Chloroplast genome Middleton et al. ( 2014 )

Genomes S b and S l 1.4 Six nuclear genes Marcussen et al. ( 2014 )

Genomes S l (of Ae.

longissima ) and S l (of

Ae. sharonensis )

0.4 Six nuclear genes Marcussen et al. ( 2014 )

Genomes S and

C,M,N,U

2,5 Six nuclear genes Marcussen et al. ( 2014 )

Genomes C,M and U 1.4 Six nuclear genes Marcussen et al. ( 2014 )

Genomes C and M 0.7 Six nuclear genes Marcussen et al. ( 2014 )

Genomes N and C, M,U 2.0 Six nuclear genes Marcussen et al. ( 2014 )

M. Feldman and A.A. Levy