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increase in expression during durum evolution and genes related to germination and
germination inhibition increased in expression in the embryo in the more recent
stages of durum evolution (Ben-Abu et al. 2014 ). Yuan et al. ( 2014 ) described dif-
ferential expression of the CENH3 genes between wild and domesticated tetraploid
wheat. Triticum araraticum ssp. armeniacum , the wild progenitor of Triticum
timopheevii subsp. timopheevii , had a higher transcript level of αCENH3 while the
domesticated subspecies had a lower expression of αCENH3 and increased expres-
sion of βCENH3. Similar changes in the CENH3 expression model were found in
wild and in domesticated types of T. turgidum ; the wild subspecies of T. turgidum
exhibited higher expression level of αCENH3 whereas in the domesticated subspe-
cies the differences in expression between αCENH3 and βCENH3 were not so obvi-
ous. In contrast to the markedly higher expression level of αCENH3 in wild
tetraploids, expression of βCENH3 was enhanced to a level near that of αCENH3 in
domesticated tetraploids (Yuan et al. 2014 ). These genome-wide analyses are
providing insight into the molecular basis of plant domestication, in particular for
the non-obvious cellular functions that were selected during evolution under
cultivation.
2.7 Concluding Remarks
Evolution in the Triticeae is a complex network of general processes and singular
events which have occurred over long periods of time and which are still happening
in a dynamic environment. Perhaps the most conspicuous feature of the evolution of
the group is the divergence of several forms at the diploid level and their conver-
gence at the polyploid one. This reticulate pattern of evolution provides the basis for
many of the characteristics of individual diploid species and the often-unclear
demarcations between closely or partially related allopolyploid species.
Although the diploid species of the wheat group are presumably descended from
a common ancestor, they have diverged considerably from one another in a rela-
tively short time (2.0 to 4.5 MYA). Cytogenetic studies have shown that almost
every species has a distinct genome. The homoeologous chromosomes of the differ-
ent genomes exhibit varying degrees of reduced affi nity for one another, and conse-
quently, they do not pair regularly at meiosis in the inter-specifi c hybrids. As a
result, almost all of the inter-specifi c hybrids are completely or almost completely
sterile and thus, the species are genetically isolated from one another. So, the diploid
level is characterized by chromosomal and genetic divergence that is expressed in
the considerable eco- geographical and morphological specialization that is evident
in the well-defi ned dispersal unit marking each diploid species.
The allopolyploids in the wheat group have experienced a multifaceted process
of evolution, in nature, through allopolyploidization and natural hybridization
between polyploids and, under domesticaton, in the case of the Triticum species.
Early cytogenetic studies showed that the polyploids of the wheat group are allopoy-
ploids. Morphologically and cytogenetically the allopolyploid species are clustered
M. Feldman and A.A. Levy