55Hexaploid wheat, that is only partially isolated from tetraploid wheat, exchanged
genes with wild emmer that grew nearby (Zohary and Brick 1961 ). Gene fl ow
between different domesticated varieties and species has also played an important
role in increasing the genetic variability. For thousands of years farmers have been
growing mixtures of different genotypes and even different cytotypes (Zeven 1980 ).
Such cytotype mixtures included representatives of two or even three different spe-
cies of domesticated wheat, namely, T. monococcum, T. turgidum (emmer and naked
tetraploid wheat) and T. aestivum (spelt and bread wheat), each represented by a
number of different genotypes, thus facilitating massive inter- and intra- specifi c
gene fl ow. These endless hybridization s during the 10,000 years of cultivation have
enriched the domesticated gene pool.
Mutations have also played an important role in increasing genetic variability.
The genetic structure of the allopolyploid species of wheat, i.e., the existence of
four (in tetraploids) or six (in hexaploids) doses of gene loci, reinforced by a diploid-
like cytological behavior and predominantly self-pollination, has proven a very
successful genetic system facilitating a rapid buildup of genetic diversity. In such
allopolyploid species the accumulation of genetic variation through mutation or
hybridization is tolerated more readily than in diploid species. Recent data show
that allopolyploidization can cause rapid genomic changes and, thereby, accelerate
their evolution (Levy and Feldman 2004 ; Feldman and Levy 2005 ). Moreover, allo-
polyploidy facilitates genetic diploidization —the process whereby existing genes in
multiple doses can be diverted to new functions. Thus, tetraploid and hexaploid
wheat can accumulate a signifi cant amount of genetic variation through mutations.
Mutations exerting a lethal or semi-lethal effect at the diploid level, such as Q, s,
(the sphaerococcum gene) C , (the compactum gene) and Ph1 , have been success-
fully utilized at higher levels of ploidy. Induction of mutations may have been accel-
erated by the activity of transposable elements (Kashkush et al. 2003 ) and
genome -restructuring genes (Feldman and Strauss 1983 ).
The activation of transposable elements, mainly retrotransposons by various
environmental and climatic stresses has an important evolutionary signifi cance. In
addition to the generation of genetic variability due to epigenetic changes, e.g.,
DNA methylation, chromatin acetylation and activity of various small RNA mole-
cules, genome restructuring may lead to the formation of new linkage groups. Rapid
chromosomal rearrangements can also contribute to the build up of partial isolating
mechanisms between differentiating genotypes in polymorphic fi elds. Activity of
genome-restructuring genes as well as transposable elements during wheat cultiva-
tion may explain the wide occurrence of chromosomal rearrangements among
domesticated wheat taxa.
Being native to the marginal Mediterranean habitats of the Fertile Crescent, wild
emmer, the wild progenitors of domesticated tetraploid wheat, was pre-adapted for
domestication. It is predominantly self-pollinated species with large and well-
protected grains that assist the safe and rapid reestablishment of the stand. Its large
seed size rendered it very attractive to the ancient gatherer. Its annual habit made it
also amenable for dry farming, while its self-pollination system could have aided
in the fi xation of desirable mutants and recombinants resulting from occasional
2 Origin and Evolution of Wheat and Related Triticeae Species