49transcripts in two lines of synthetic allohexaploid wheat and found that 7 % of the
genes had nonadditive expression, while Akhunova et al. ( 2010 ) found in synthetic
allohexaploid wheat that about 19 % of the studied genes showed nonadditive
expression. Li et al. ( 2014a , b ) found that nonadditively expressed protein-coding
genes were rare but relevant to growth vigor. Moreover, a high proportion of pro-
tein-coding genes exhibited parental expression level dominance. Similar studies
by He et al. ( 2003 ) showed that the expression of a signifi cant fraction of genes
(7.7 %) was altered in the synthetic allohexaploid T. turgidum-Ae. tauschii , and that
Ae. tauschii genes were affected much more frequently than those of T. turgidum.
Interestingly, silencing of the same genes was found also in natural hexaploid
wheat, indicating the reversibility of the effect and that the regulation of gene
expression is established immediately after allohexaploidization and maintained
over generations (He et al. 2003 ; Chague et al. 2010 ). In accord with these results,
increased small interfering RNA density was observed for transposable element–
associated D ho moeologs in the progeny of newly formed allohexaploid wheat,
which may account for biased repression of D homoeologs (Li et al. 2014a , b ). It is
of interest to note that several genes, that are silent in the parental species, became
active in the newly formed allohexaploid (He et al. 2003 ). Similarly, cDNA-AFLP
gels also revealed several cDNAs that were expressed only in the allopolyploids
and not in the diploid progenitors (Shaked et al. 2001 ; Kashkush et al. 2002 ).
The rapid processes of cytological and genetic diploidization allow for the devel-
opment and occurrence of two contrasting and highly important genetic phenomena
in allopolyploid wheat that contribute to their evolutionary success: (1) build up and
maintenance of enduring inter-genomic favorable genetic combinations (inter-
genomic heterosis), and (2) genome asymmetry in the control of a variety of mor-
phological, physiological and molecular traits, i.e., complete or principal control of
certain traits by only one of the constituent genomes. However, while the fi rst phe-
nomenon was taken for granted by plant geneticists, genomic asymmetry was only
recently documented in allopolyploids of the wheat group (Peng et al. 2003a , b ;
Fahima et al. 2006 ; Feldman and Levy 2009 ; Feldman et al. 2012 ). The Phenomenon
of genome asymmetry is manifested in a clear-cut division of tasks between the
constituent genomes of allopolyploid wheat (Levy and Feldman 2004 ; Feldman and
Levy 2009 ; Feldman et al. 2012 ). Genome A controls morphological traits while
genome B in allotetraploid wheat an d genomes B and D in allohexaploid wheat
control the reaction to biotic and abiotic stresses and regulate the adaptation to eco-
logical conditions. Similarly, Li et al. ( 2014a , b ) reported that genes for which the
total homoeolog expression level in the progeny of newly formed allohexaploid
wheat was similar to that in T. turgidum potentially participating in development
and those with similar expression to that in Ae. tauschii involved in adaptation.
Inter-genomic pairing would have lead to disruption of the linkage of the
homoeoalleles that contribute to positive inter-genomic interactions and, as well as,
lead to segregation of genes that participate in the control of certain traits by a single
genome. Inter-genomic recombination may result therefore, in many intermediate
phenotypes that may affect, in a negative manner, the functionality, adaptability and
stability of the allopolyploids.
2 Origin and Evolution of Wheat and Related Triticeae Species