63in three natural groups: one sharing the U genome of Ae.umbellulata , one the D
genome of Ae. tauschii , and one the A genome of T. urartu. The A-genome cluster
includes all the domesticated forms of wheat. The species of each group share one
common genome and differ in their other genome in tetraploids and their two other
genomes in hexaploids. In basic morphology and particularly in the structure of the
seed dispersal unit, the allopolyploids of each group resemble the diploid donor of
the common genome and differ in features of the other genomes. This genomic
structure accounts for the relatively high rate of successful gene fl ow between the
allopolyploids that belong to one cluster resulting in the production of new recom-
binant genome s. Hence, genomes that are isolated at the diploid level are able to
exchange genes and recombine at the polyploid one.
The above reviewed studies indicate that in the wheat group the process of allo-
polyploidization generates a genetic shock that induces a burst of genetic and
epigenetic, non-Mendelian, alterations, (referred to as revolutionary changes), some
of which could not occur at the diploid level. Some of these changes presumably
improve the ability of the newly formed allopolyploids to survive in nature and to
compete with their parental species. Transposable elements seem to play a signifi -
cant role in the various responses to allopolyploidization due to their abundance and
also due to their tendency to be dysregulated as a result of genomic shocks.
Other changes occur sporadically over a long time period during the life of the
allopolyploid species (evolutionary changes). From a population point of view, the
chances of a new individual, such as a nascent hybrid/allopolyploid, to establish
itself as a new species is almost null, unless it has some increased fi tness over its
parents. This must happen within a few generations otherwise the new species will
rapidly be extinct. The revolutionary changes described here may contribute to the
establishment of the new species. Instantaneous elimination of sequences from one
genome in the newly formed allopolyploids increases the divergence of the
homoeologous chromosomes and thus, leads to exclusive intra-genomic pairing that
improves fertility. Mechanisms such as loss of deleterious genes (e.g. genetic
incompatibilities), or positive dosage effects or new inter-genomic heterotic interac-
tions may all rapidly increase the fi tness of the nascent species. The evolutionary
changes, on the other hand, contribute to the build up of genetic variability and thus,
increase adaptability, fi tness, competitiveness, and colonizing ability.
Altogether, the reported revolutionary and evolutionary genomic changes,
emphasize the dynamic plasticity of the wheat allopolyploid genome with regards
to both structure and function. Presumably these changes have improved the adapt-
ability of the newly formed allopolyploids and facilitated their rapid colonization of
new ecological niches. No wonder, therefore, that domesticated allopolyploid
wheats exhibit a wider range of genetic fl exibility than diploid wheats and could
adapt themselves to a great variety of environments.
The evolutionary relationships of the Aegilops , Amblyopyrum and Triticum spe-
cies are not confi ned to the generic boundaries of the wheat group. It is obvious
from the ability of chromosomes of alien species generally to substitut e for the
chromosomes of only one homoeologous group that homoeologous relationships
exist between the chromosomes of the wheat group and other genera in the Triticeae.
2 Origin and Evolution of Wheat and Related Triticeae Species