39the insertion dates of TEs that the majority of differential proliferation of TEs in the
B and A genomes of bread wheat (87 and 83 %, respectively), leading to a rapid
sequence divergence, occurred in these genomes already at the diploid level, prior
to the allotetraploidization event that brought them together in Triticum turgidum,
about 0.5 MYA. Finally, rewiring of gene expression in hybrids might dysregulate
the silencing of transposons, resulting in activation of transposons, in reduction of
the hybrid fi tness or viability and thus might contribute to speciation (Levy 2013 ).
To sum up, the diploid species of the wheat group underwent a reticulated evolu-
tion. The divergence of the S and A lineages established the basal branches of the
wheat group. The time of this divergence determines the divergence of the group.
Amblyopyrum muticum diverged presumably earlier since it has many ancestral
traits, is allogamous (self-incompatible), and intermediate in morphology between
Aegilops and Agropyron (Eig 1929a , b ; Ohta 1991 ). Aegilops speltoides exhibits
ancestral traits and is also allogamous. Divergence of D genome from S and A
genome s was presumably through inter-generic hybridization. Steuernagel et al.
( 2014a , b ) suggested that Ae. sharonensis might be a product of hybridization
between Ae. speltoides and Ae. tauscii. Yet Ae. longissima is closer to Ae. tauschii
than Ae. sharonensis. Ae. sharonensis might be the result of hybridization between
Ae. lomgissma and Ae. bicornis (Waines and Johnson 1972 ). Ae. bicornis presum-
ably derived from Ae. speltoides via mutations and TEs activities. Ae. searsii is the
most advanced species in Emarginata because it has several advanced morphologi-
cal traits. Ae. caudata diverged from Ae. searsii or from hybridization between Ae.
searsii and Ae. tauschii. Ae. comosa , Ae. Uniaristata, and Ae. umbellulata derived
from Ae. caudata.
Because of the fairly recent speciation of the diploid species, most of them have
relatively limited morphological and molecular variation, occupy only a few well-
defi ned ecological habitats, and are distributed throughout relatively small geo-
graphical areas (Eig 1929a ; Zohary and Feldman 1962 ; Kimber and Feldman 1987 ;
Van Slageren 1994 ).
2.5 Evolution of the Allopolyploid Species
The discovery of the accurate chromosome numbers of the different Triticum species
(Sakamura 1918 ) showed that this genus comprises a polyploid series based on x = 7,
containing diploids, tetraploids, and hexaploid species. Soon after, studies of chro-
mosomal pairing in hybrids between species of different ploidy levels disclosed that
the polyploids are allopolyploids, i.e., each polyploid species was formed by inter-
specifi c or inter-generic hybridization followed by chromosome doubling (Kihara
1919 , 1924 ; Percival 1921 ; Sax 1921 , 1927 ). These studies showed that tetraploid
wheat contains one genome, A, that derived from a diploid Triticum species and a
second genome, B, from another, yet unknown diploid species. Hexaploid wheat con-
tains the two genomes of tetraploid wheat, A and B, and a third genome, D, from
another species. The conclusion that the polyploids are allopolyploids was also
2 Origin and Evolution of Wheat and Related Triticeae Species