38
species (as male) and backcrossing of the resulting hybrid(s) with the ancestor of
Triticum-Aegilops. Fan et al. ( 2013 ) suggested that Agropyron elongatum va r
bessarabicum ( Thinopyrum bessarabicum , Elymus farctus subsp. bessarabicus ) is
close to species of Aegilops section Sitopsis and consequently might have hybrid-
ized and introgressed with them.
Transposable elements (TEs) have the potential to affect genome structure and
function through transposition, ectopic recombination and epigenetic repatterning
(Bennetzen 2005 ; Slotkin and Martienssen 2007 ; Fedoroff 2012 ). They may mutate
genes, alter gene regulation, and generate new genes allowing response to environ-
mental challenges thus providing fuel for evolution (Kidwell and Lisch 2000 ).
Moreover, TEs have served as building blocks for epigenetic phenomena, both at
the level of single genes and across larger chromosomal regions (Slotkin and
Martienssen 2007 ). Since the variation that is induced by TEs depends on their
activity that is governed by epigenetic regulation (Slotkin and Martienssen 2007 ),
their activation might be induced by genetic and environmental stresses (Fedoroff
2012 ). Senerchia et al. ( 2013 ) suggested that ancestral TE families followed inde-
pendent evolutionary trajectories among related species, highlighting the evolution
of TE populations as a key factor of genome differentiation. The balance between
genome expansion through TE proliferation and contraction through deletion of TE
sequences drives variation in genome size and organization (Bennetzen and Kellogg
1997 ). Hence, the large differences in genome size between the various diploid spe-
cies of the wheat group (Eilam et al. 2007 ; Table 2.5 ) suggest that TE activity has
played an important role in the genomic evolution of these species. Indeed, Yaakov
et al. ( 2013 ) determined the relative copy numbers of 16 TE families in these spe-
cies and found high variation and genome-specifi city of TEs, indicating the appar-
ent involvement of TEs in the evolution of the diploid species. Similarly, Ben-David
et al. ( 2013 ) determined the copy number of short interspersed nuclear elements
(SINEs) of the wheat family Au SINE in these diploid species and found that SINEs
may play a prominent role in the genomic evolution of these species through stress-
induced activation. Hence, the variation in copy number of TEs among the diploid
species of the wheat group may imply that the main genomic differences between
these species are the results of differential activity of TEs (Yaakov et al. 2013 ).
Indeed, TEs, accounting for a very large fraction of the genome s of the diploid
species of the wheat group, [80 % of well-annotated TEs, with a majority of LTR
retrotransposons (Senerchia et al. 2013 )], were found to be one of the main drivers
of genome divergence and evolution in this group (Yaakov et al. 2012 ). Middleton
et al. ( 2013 ) also found that several TE families differ strongly in their abundance
between the diploid species of this group, indicating that TE families can thrive
extremely successfully in one species while going virtually extinct in another.
Several TE families have undergone either proliferation or a reduction in abundance
during species diversifi cation. Using miniature inverted- repeat transposable element
(MITE) markers, Yaakov et al. ( 2012 ) assessed genetic diversity among several dip-
loid species, namely, Ae. speltoides. Ae. searsii. Ae. sharonensis , Ae.longissima , Ae.
tauschii , and T. urartu , and found that about 80 % of the markers showed polymor-
phic insertions among species and accessions. Charles et al. ( 2008 ) estimated from
M. Feldman and A.A. Levy