1516.6.4 Cytoplasmic Effects
The infl uence of the cytoplasm on male fertility as well as several other morphologi-
cal traits is well established in a number of plant species, including wheat (Tsunewaki
et al. 2002 ). Analyses of meiotic pairing in wheat × rye hybrids obtained from allo-
plasmic wheat lines have revealed that alien cytoplasmic factors may also infl uence
the homoeologous pairing control system of hexaploid wheat.
Wang et al. ( 1999 ) examined chromos ome pairing at metaphase I in wheat × rye
hybrids obtained from 16 alloplasmic lines of Chinese Spring. In each of the allo-
plasmic lines, the wheat cytoplasm had been replaced by the cytoplasm of a differ-
ent Triticum o r Aegilops species. This study revealed a signifi cant homoeologous
pairing promoter effect of the cytoplasms of T. timopheevii , Ae. sharonensis , Ae.
juvenalis , Ae. crassa , Ae. variabilis , whereas the cytoplasms of Ae. bicornis and Ae.
kotschyi decreased the level of pairing. However, such effects appeared to be nuclear
genotype-dependent since Wang et al. ( 1991 ), who used a different rye parent,
reported some reduction of the MI pairing by the cytoplasms of Ae. variabilis , Ae.
crassa , and Ae. juvenalis. Besides, the cytoplasms of Ae. kotschyi and Ae. crassa
showed opposite effects in hybrids with rye (ABDR) and with hexaploid tri ticale
(AABBDR) (Wang et al. 1999 ).
The infl uence of Ae. crassa , Ae. squarrosa and T. dicoccum cytoplasms in
wheat × rye hybrids has been examined in a more recent analysis (Kussovska et al.
2009 ) using alloplasmic wheat lines derived from varieties Aurora and Rusalka,
which were crossed with two distinct rye inbred lines. In the four ABDR nuclear
genotype combinations, the hybrids with the Ae. crassa cytoplasm showed a rela-
tively high level of chiasmata compared with the euplasmic hybrids, which supports
a homoeologous pairing promoter effect of the Ae. crassa cytoplasm. The ability of
the Ae. crassa cytoplasm to disrupt the meiotic control system of wheat was shown
also in an earlier report (Sechnyak and Simonenko 1991 , cited by Simonenko et al.
1998 ). The effect of the cytoplasms of Ae. squarrosa and T. dicoccum is conditioned
by the wheat genotype since the genetic background from cultivar Rusalka was
permissive for the promoting homoeologous pairing of the alien cytoplasms while
no signifi cant deviations from the euplasmic hybrids were found in the set of hybrids
derived from the Aurora alloplasmic lines (Kussovska et al. 2009 ).
A number of evidence support that the function of nuclear gene products can be
affected by the expression of extranuclear genes (Hanson and Bentolila 2004 ).
Interaction between nuclea r and cytoplasmic genes seems a likely explanation for
the disparate alien cytoplasm effect patterns reported above.
6.7 Homoeologous Pairing Control in Other Allopolyploid
Species
Wheat is, by far, the polyploid species where the genetic system ensuring a diploid-
like meiotic behavior has been more extensively explored and then characterized.
However, there are other allopolyploids, including some important crops, whose
6 The Mode and Regulation of Chromosome Pairing...