| Most interspecific hybrids from crosses between species with well-differentiated
genomes are highly sterile due to interactions between the specific genes
of the parental speceis, which complement sterility due to meiotic irregularities.
Only natural or induced amphidiploids and some of the progenies from backcrosses
with parental species or from out-crosses with other related species are
fertile, and these progenies can be used for the analysis of the hybrid
sterility components. Sterility may be due to hemizygosity of the species-specific
genes in the F1, and fertility is restored when these genes become
homozygous in the amphidiploid, or sterility genes from one of the parents
may be eliminated in certain fertile backcross progenies. If parental species
differ cytoplasmically, then cytoplasm-specific nuclear genes from the genome
of the cytoplasm donor species may be transferred to the chromosomal complement
of the recurrent male parent which usually is T. aestivum or T.
durum. The use of polyploid T. aestivum facilitates the transfer
of the alien genes to wheat chromosomes because a greater variety of the
aneuploid gametes and zygotes produced are viable and functional, and the
effects of aneuploidy and sterility due to meiotic irregularities are minimized.
Of course, only those alien genes which are expressed in the genetic background
of T. aestivum would be identified and examined. Others, which are
not transferred to wheat, or remain unexpressed, or those having minor effects
will remain unidentified. Genes controlling most easily noticeable cytoplasmic
effects like restoration of male fertility and vigor of hybrid plants are
examined most often. In most species, the genes controlling restoration
of male fertility and plant vigor appear to be closely linked or are located
on the same arm of a critical chromosome of the cytoplasm donor species
(MAAN, 1977b). The presence of the alicn chromosome in wheat plants can
be readily detected when derived hybrid progenies are crossed with alloplasmic
wheat lines having cytoplasm of the same donor species from which the critical
nuclear genes were extracted, and F1's are cytologically examined
for lack of pairing between wheat and alien chromosome(s) and from other
phenotypic effects. |