46
microscopic deletion in the middle of chromosome arm 5BL (Jampates and Dvorak
1986 ). Mapping data suggested that Ph1 might encode for a gene that affects cell
cycle progression through the regulation of cyclin-dependent kinases (Griffi ths
et al. 2006 ; Al-Kaff et al. 2008 ; Yousafzai et al. 2010 ), however, direct evidence
through complementation studies is still missing. Recently, Bhullar et al. ( 2014 )
identifi ed a candidate gene in the Ph1 region that is different from the one that was
proposed previously (Griffi ths et al. 2006 ; Al-Kaff et al. 2008 ; Yousafzai et al.
2010 ). Virus-induced silencing of this newly identifi ed gene disrupts the alignment
of chromosomes on the metaphase plate in early stages of meiosis suggesting a role
of microtubules in its function (Bhullar et al. 2014 ). This in accord with the fi nding
of Avivi and Feldman ( 1973 , and reference therein) and Vega and Feldman ( 1998 )
who found that Ph1 affects the interaction between the centromeres and microtu-
bules, possibly through a microtubule-associated protein (MAP) whose activity is
located near the centromere.
To sum up, cytological diploidization in allopolyploid Triticum was engendered
by two independent, complementary systems. One is based on the physical diver-
gence of chromosomes, and the second, on the genetic control of pairing. The
Ph- gene system superimposes itself on and takes advantage of—and thereby rein-
forces—the above-described system of the physical differentiation of homoeolo-
gous chromosomes. In addition, stringent selection for fertility in the allopolyploid
Triticum species might well favor the development of two systems to effect the
suppression of multivalent formation and the promotion of bivalent pairing in nature
and, more so, in domesticated material.
The process of cytological diploidization in the allopolyploid species of the wheat
group has been critical for their successful establishment in nature. The restriction
of pairing to completely homologous chromosomes ensures regular segregation of
genetic material, high fertility, genetic stability, and disomic inheritance that pre-
vents the independent segregation of chromosomes of the different genome s. This
mode of inheritance leads to permanent maintenance of favorable inter- genomic
genetic interactions, thus, enables to fi x heterotic interaction between genomes. On
the other hand, disomic inheritance sustains the asymmetry in the control of many
traits by the different genomes (Feldman et al. 2012 ). In addition, since cytological
diploidization facilitates genetic diploidization , existing genes in double and triple
doses can be diverted to new functions through mutations, thereby giving preferen-
tiality to the creation of favorable, new inter-genomic combinations.
How the two or three divergent genome s present in a single nucleus of an allo-
polyploid were driven to operate in a harmonious manner. From one hand, homoeo-
alleles in allopolyploid wheat may differ from one another by allelic variation, and
in this case, activity of all the duplicated genes may produce desirable inter-genomic
interactions and heterotic effects. Inter-genomic gene interactions may be, in some
cases, expressed in novel traits that do not exist in their parental diploids. Some of
these traits may have great adaptive value. Inter-genomic gene interactions have
direct relevance also to wheat cultivation. For example, the baking quality of allo-
hexaploid wheat (bread wheat) is due to the unique properties of its gluten—a prod-
M. Feldman and A.A. Levy