Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

54

T. aestivum to 0.28–0.47 MYA (Marcussen et al. 2014 ) (Table 2.10 ). They suggested
that the age of the time of the allopolyploidization events might be older than previ-
ously suggested. Since the hybridization between the donor of the B genome and T.
urartu leading to the formation of allotetraploid wheat was soon after the diver-
gence of T. urartu , i.e., 0.36 to 0.7 MYA (Huang et al. 2002b ; Dvorak and Akhunov
2005 ; Marcussen et al. 2014 ) and that of T. aestivum about 0.01 MYA (Feldman and
Levy 1995 ; Feldman 2001 ), the earlier divergence time between the genomes of the
diploid species and those of the polyploid ones indicate that the intra-specifi c diver-
gence has occurred at the diploid level, namely, in Ae. speltoides, T. monococcum/
urartu and Ae. tauschii. The divergence of the S genome of Ae. speltoides and the B
genome of polyploid wheat might have been followed by a speciation and if so, the
donor of the B genome is either extinct or yet undiscovered extant species.
Whereas the diploid species of the group have evolved in the Pliocene the tetra-
ploids have been produced later on, in the Pleistocene (Table 2.3 ). The polyploids
species of Aegilops were produced in the east Mediterranean region and have spread
out from this center (Kihara 1954 ).

2.6 Evolution of Wheat Under Cultivation

During the fi rst several millennia of cultivation genotypes with nonfragile spikes of
einkorn and emmer gradually replaced the wild genotypes (Feldman and Kislev
2007 ). It is assumed that the mutants that founded domesticated einkorn and emmer
happened in a small number of wild plants. Similarly, the number of hexaploid
plants produced by independent hybridization events between domesticated tetra-
ploid wheat and Ae. tauschii was limited. Hence, the primitive domesticated
Triticum species contained a narrow and very restricted genetic variability. Yet, the
initial nonbrittle einkorn ( monococcum ) and emmer plants grew for a long period in
mixed populations with wild forms (Tanno and Willcox 2006 ; Kislev 1984 ; Feldman
and Kislev 2007 ) and presumably exchanged genes with them. Moreover, even after
the complete replacement of wild emmer by domesticated emmer, the latter could
continued to introgress with wild genotypes that grew in its vicinity and thereby,
broadened its genetic basis (Huang et al. 1999 ; Dvorak et al. 2006 ; Luo et al. 2007 ).

Table 2.10 Beginning-divergence time of the genome s of diploid species and the homologous
genomes of Triticum aestivum in million years ago

Genomes

Beginning

divergence time

Type of DNA

sequences studied Reference

Genome A of T. urartu and

genome A of T. aestivum

0.58–0.82 Several hundreds

nuclear genes

Marcussen et al.

( 2014 )

Genome S of Ae. speltoides

and genome B of T. aestivum

0.98 Chloroplast genome Middleton et al.

( 2014 )

Genome D of Ae. tauschii and

genome D of T. aestivum

0.28–0.47 Several hundreds

nuclear genes

Marcussen et al.

( 2014 )

M. Feldman and A.A. Levy