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Allopolyploid species undergo structural genomic changes also during the lifetime
of the taxon (evolutionary changes) that generate a new v ariation that could not take
place in the diploid parental genomes and that occur almost exclusively in an allo-
polyploid background. Allopolyploids harboring two or more different genomes
within each nucleus, may facilitate inter-genomic horizontal transfer of chromo-
somal segments, transposable elements or genes between the constituent genomes.
Inter-genomic invasion of chromatin segments from the B genome into the A genome
was demonstrated by FISH analysis (Belyayev et al. 2000 ). Cytogenetic studies have
shown that inter-genomic translocations occur in the allopolyploids of the wheat
group (Maestra and Naranjo 1999 , and reference therein). Moreover, in contrast to
diploids, which are genetically isolated from each other and have undergone diver-
gent evolution, allopolyploids in the wheat group exhibit convergent evolution
because they contain genetic material from two or more different diploid genome s
and can exchange genes with each other via hybridization and introgression , result-
ing in the production of new genomic combinations (Zohary and Feldman 1962 ).
The presence of duplication or triplication of the genetic material in tetraploids
and hexaploids, respectively, has relaxed constraints on the function of the multiple
genes enabling, in the long run, continued genetic diploidization such as silencing
of one of the duplicated or triplicated genes or divergence of one homoeologous
locus to a new function. Thus, the accumulation of genetic variation through muta-
tions is tolerated more readily in allopolyploid than in diploid species (Dubcovsky
and Dvorak 2007 a, b).
Note that evolutionary changes might also occur in an accelerated manner, thanks to
the buffering of mutations in the polyploid background (Mac Key 1954 , 1958 ; Sears
1972 ; Dubcovsky and Dvorak 2007 a, b), leading to rapid neo- or subfunctionalization
of genes and to a further process of diploidization and of divergence from the diploid
progenitor genomes. Akhunov et al. ( 2013 ) uncovered a high level of alternative splic-
ing pattern divergence between the duplicated homeologous copies of genes in common
wheat. Their results are consistent with the accelerated accumulation of alternative
splicing isoforms, nonsynonymous mutations, and gene structure rearrangements in
the wheat lineage, likely due to genetic redundancy created by allopolyploidization
(Akhunov et al. 2013 ). Whereas these processes mostly contribute to the degeneration
of a duplicated genome and its diploidization, they have the potential to facilitate the
origin of new functional variations, which, upon selection in the evolutionary lineage,
may play an important role in the origin of novel traits (Akhunov et al. 2013 ).
According to Stebbins ( 1950 ), newly formed allopolyploids are often character-
ized by limited genetic variation, a phenomenon he referred to as the “polyploidy
diversity bottleneck.” This bottleneck arises because only a few diploid genotypes
were involved in the allopolyploid formation events, because the newly formed allo-
polyploid is immediately isolated reproductively from its two parental species and
because time was not suffi cient for the accumulation of mutations. Despite this diver-
sity bottleneck and despite the fact that all Aegilops and Triticum allopolyploids were
formed later than their ancestral diploids [e.g., tetraploid wheat about 300,000–
500,000 years ago (Huang et al. 2002a ) and allohexaploid wheat was formed only
about 10,000 years ago (Feldman et al. 1995 ; Feldman 2001 )], they display a greater
M. Feldman and A.A. Levy