Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

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responsible for the eco-geographical adaptation of the various allopolyploid species
in each cluster. Thus, inter-specifi c hybridization has played a decisive role in the
production of a wide range of genetic variation in the allopolyploid species and
probably signifi cantly contributed to their evolutionary success.
Huang et al. ( 2002b ), Dvorak and Akhunov ( 2005 ), and Marcussen et al. ( 2014 ),
based on sequencing of nuclear genes, proposed that the wild emmer, T. turgidum
subsp. dicoccoides , was formed about 360,000–700,000 years ago (Table 2.9 ).
Gornicki et al. ( 2014 ) , based on sequencing of chloroplast genomes, concluded that
Aegilops speltoides forms a monophyletic clade with the allopolyploid Triticum spe-
cies emmer and timopheevii , which originated within the last 0.7 and 0.4 MYA,
respectively (Table 2.9 ). Mori et al. ( 1995 ), using RLFP analysis of nuclear genes,
assumed that wild timopheevii , T. timopheevii subsp. armeniacum , was formed
about 400,000 years ago. The geographic distribution of chloroplast haplotypes of
the wild tetraploid wheats and A. speltoides illustrates the possible geographic origin
of the Emmer lineage in the southern Levant and the timopheevi lineage in northern
Iraq (Gornicki et al. 2014 ). This is in accord with the fi nding of Dvorak (personal
communication) that wild emmer was formed in the vicinity of Mt Hermon. Aegilops
speltoides is the closest relative of the diploid donor of the chloroplast and the
nuclear genome s to emmer and timopheevii lineages (Gornicki et al. 2014 ).
Middleton et al. ( 2014 ) estimated that the B genome donor to allopolyploid
wheat diverged from Ae. speltoides approximately 0.98 MYA (Table 2.10 ). The
divergence time of genome A m of T. monococcum and A of T. urartu is 0.57 MYA
(Middleton et al. 2014 ), that between the A genome of T. urartu and A genome of
T. aestivum is 0.58–0.82 MYA and that of D genome of Ae. tauschii and D of

Table 2.9 Time of formation of the allopolyploid species of Triticum in million years ago

Species

Time of

formation Method of study Reference

Triticum turgidum

subsp. dicoccoides

(wild emmer)

<0.500 Nucleotide sequences of two

nuclear genes

Huang et al. ( 2002b )

0.360

(0.190–

0.540)

Nucleotide sequences of four

nuclear genes

Dvorak and Akhunov

( 2005 )

<0.800 Nucleotide sequences of several

hundreds nuclear genes

Marcussen et al.

( 2014 )

0.700 Sequencing chloroplast DNA Gornicki et al. ( 2014 )

Triticum timopheevii

subsp. armeniacum

(wild timopheevii)

0.050–0.300 RFLP analysis of nuclear DNA Mori et al. ( 1995 )

0.400 Sequencing Chloroplast DNA Gornicki et al. ( 2014 )

Triticum aestivum

subsp. aestivum

(common wheat)

0.008 Nucleotide sequences of two

genes

Huang et al. ( 2002b )

0.008 Nucleotide sequences of four

nuclear genes

Dvorak and Akhunov

( 2005 )

<0.400 Nucleotide sequences of several

hundreds nuclear genes

Marcussen et al.

( 2014 )

2 Origin and Evolution of Wheat and Related Triticeae Species