37Ae. longissima and Ae. bicornis are closely related, and that Ae. sharonensis is equally
close to Ae. bicornis and Ae. longissima. Aegilops searsii is close to Ae. longissima
( Feldman et al. 1979 ), and i s the only Emaginata species with advanced traits such as
free grains, length of glumes close to the length of fl orets and short culms. Ae. searsii,
being the advanced species in this group, diverged from the other Emarginata species
about 1.0 to 2.0 MYA (Marcussen et al. 2014 ). Ae. bicornis diverged from Ae. longis-
sima about 1.4 MYA and Ae. sharonensis diverged from Ae. longissima and Ae. bicor-
nis about 0.4 MYA (Marcussen et al. 2014 ). Ae. longissima has spikes that fall entire
at maturity but, later on, during the summer, may disarticulate into spikelets like that
of Ae. tauschii (the barrel type) indicating its relationships to Ae. tauschii. Ae. sharo-
nensis , the close relative of Ae. longissima , was recently suggested to be closer to Ae.
tauschii or its progenitor than to Ae. speltoides (Steuenagel et al. 2014a , b ). This was
proposed from sequencing the genome of two accessions of Ae. sharonensis and
sequencing the transcriptomes of 16 accessions that cover the natural habitat of Ae.
sharonensis (Steuenagel et al. 2014a , b ). Consequently, they concluded that the
Sitopsis classifi cation is inconsistent with their genome -wide analysis of Ae. sharo-
nensis , provoking the need to reconstruct the current taxonomy of Aegilops.
How the diploid species of this group evolved? Speciation at the diploid level
might result from accumulation of mutations in coding and in noncoding sequences
as well as structural changes that lead to the buildup of genetic barriers between the
diverging taxa. Feldman and Strauss ( 1983 ) described a genome-restructuring gene
in Ae. longissima that produced a large number of chromosomal rearrangements in
plants homozygous for it. Genome restructuring is an ongoing process in natural
plant populations of Ae. speltoides (Belyayev 2013 ). Numerical chromosomal aber-
rations, spontaneous aneuploidy, B-chromosomes, and repatterning and reduction
in the species-specifi c tandem repeats have been detected in marginal populations of
Ae. speltoides (Raskina et al. 2004 ; Belyayev et al. 2010 ) indicating that chromo-
somal re-patterning might be one mechanisms of plant genome evolution and spe-
ciation (Raskina et al. 2004 ). Such genetic, epigenetic and structural changes might
have promoted the formation of genetic barriers between the diverging species that
resulted in sterility of the inter-specifi c or inter-generic hybrids.
Other species might evolve through inter-specifi c or even inter-generic hybrid-
ization. Judging from the patterns of seed proteins, Waines and Johnson ( 1972 )
suggested that Ae. sharonensis was originated from hybridization between Ae.
longissima and Ae. bicornis. Recent comparisons of nuclear genes indicated that an
ancestral D lineage was derived from hybridization between ancient A and S lin-
eages (Marcussen et al. ( 2014 ). Li et al. ( 2014a , b ), reevaluating the origin of Ae.
tauschii by using recently published data from sequencing of nuclear DNA
(Marcussen et al. 2014 ) and chloroplast DNA (Gornicki et al. 2014 ) as well as addi-
tional data, confi rmed the hybrid origin of the D-genome clade but concluded that
this clade have had a more complex origin, one that may have involved multiple
rounds of hybridizations. Nakamura et al. ( 2009 ) studied in a number of species in
the group the PolA1 gene that codes the largest subunit of RNA polymerase I and
concluded that the Sitopsis species might have originated by an ancient hybridiza-
tion between an ancestral Triticum- Aegilops species (as female) and an Hordeum
2 Origin and Evolution of Wheat and Related Triticeae Species