35of telomeric and subtelomeric tandem repeats (Salina et al. 2006 ), are consistent with
this division of section Sitopsis. Brody and Mendlinger ( 1980 ) found that T. mono-
coccum s. lat. is close to the Sitopsis species; Ae. comosa , Ae. uniari stata and Ae.
umbellulata form a second subgroup with Ae. caudata most closely related to these
species and that Ae. tauschii is equally related to all of the species.
Phylogentic relationships of the diploid species were studied on the molecular
level in which plasmon [plastome (chloroplast genome) and chondriome (mito-
chondrial genome)] were analyzed and compared (Tsunewaki 2009 ; Kawahara
2009 ). Tsunewaki ( 2009 ) reviewed such studies in the various diploid species of the
wheat group and concluded that they exhibit a great diversifi cation. A mblyopyrum
muticum and Ae. speltoides, the two outbreeding species, showed especially clear
intra-specifi c chloroplast and mitochondrial differentiation. Earlier studies revealed
two types of electromorphs of the rubisco large subunit (the chlotoplast subunit),
H- and L-type, in the Triticum- Aegilops complex (Chen et al. 1975 ; Hirai and
Tsunewaki 1981 ). The H-type large subunit was found in the chloroplast of Ae.
speltoides (and also in that of allopolyploid Triticum species) while the L-type large
subunit distributed among the chloroplast of all diploid Aegilops a nd Triticum
species (Hirai and Tsunewaki 1981 ).
DNA sequencing has had a dramatic effect on the fi eld of molecular phylogenetics.
Such studies in the wheat group, based on data from nuclear DNA sequences (genes
or repetitious DNA) and chloroplast DNA sequences, show signifi cant inconsistencies
possibly due to ancient and recent inter-specifi c and inter-generic hybridization s and
introgression s (Kawahara 2009 ). Incongruence between chloroplast and nuclear
genomic data was often detected (Sasanuma et al. 2004 ; Kawahara 2009 ; Li et al.
2014a , b ). Monophyletic origin of Aegilops and Triticum was inferred from some of
the studies (e.g., Hsiao et al. 1995 ; Kellogg and Appels 1995 ; Kellogg et al. 1996 ;
Huang et al. 2002a , b ) whereas a polyphyletic origin of the group was deduced from
other studies (Petersen and Seberg 1997 , 2000 ; Seberg and Frederiksen 2001 ; Sallares
and Brown 2004 ; Mason-Gamer 2005 ; Petersen et al. 2006 ). It is probable that inter-
generic hybridizations and introgression s form other genera of the Triticeae e.g.,
Agropyron , blurred and obscured the monophyletic origin of the wheat group.
Several recent estimates of the divergence time of the basal genome s of the wheat
group indicated that the divergence of the A and the S genomes from an ancestral
Triticineae genome established the basal lineages of the wheat group while the
D-genome diverged from these two genomes somewhat later (Dvorak and Akhunov
2005 ; Marcussen et al. 2014 ; Gornicki et al. 2014 ). Results of molecular analyses
have shown that genomes S, A, and D are much more closely related to each other
than to other genomes (Monte et al. 1993 ; Dvorak and Zhang 1990 ; Dvorak et al.
1998 ). Marcussen et al. ( 2014 ) proposed that a homoploid ancient hybridization
event occurred long before the divergence of the diploid species of Aegilops and
indicated that the use “D-genome lineage” in their publication is nonsynonymous
with the D genome of Ae. tauschii but rather, refers to the entire D + S + M clade.
They suggested that these ancestral genomes have diverged from a common ances-
tor and from one another about 6.5 million years ago (MYA) (Table 2.7 ). Huang
et al. ( 2002b ) estimated that theses diploid species diverged from one another much
2 Origin and Evolution of Wheat and Related Triticeae Species