| Results of an analysis of the meiotic chromosome configuration of zhukovskyi
(Table 2) closely paralleled the findings of UPADHYA
and SWAMINATHAN (1963). The total number of chromosomes paired is very similar,
but there are small increases in the number of multivalents in the present
example. The observation of occasional quinquevalents and sexivalents in
our data most probably represents the translocations inferred from the trivalents
and quadrivalents seen in the hybrid with rye. Also the frequency of trivalents
and quadrivalents in zhukovskyi is lower than that expected from
the frequency of bivalents in the hybrid with rye. It is possible that there
is some selection pressure towards increased bivalent frequency in zhukovskyi,
and it could be anticipated that this would be of selective advantage. If
this reduction in multivalent formation had a genetic basis and was not
simply a physical or spatial limitation on synapsis, it could represent
an early stage in the cytological diploidization of the species. It is also possible that T. timopheevii var. zhukovskyi carries a regulator of chromosome pairing similar, if not identical, to that in T. aestivum. FELDMAN (1966) demonstrated the presence of such a mechanism in T. timopheevii, and assuming this species is involved in the evolutionary history of zhukovskyi, then it is reasonable to assume that the same allele would be found in zhukovskyi also. UPADHYA and SWAMINATHAN (1965) investigated hybrids of zhukovskyi with T. aestivum with and without chromosome 5B and concluded that zhukovskyi did not have a gene equivalent to that on chromosome 5B of T. aestivum. However, there was relatively little difference between the meiotic configurations of the two hybrids. In the 5B deficient cross there were only 3.02 more chromosomes paired than in the hybrid which included chromosome 5B. SHANDS and KIMBER (1973) also observed the meiotic configuration of a hybrid T. timopheevii var. zhukovskyi x T. aestivum, and this had only 0.09 paired chromosomes per cell less than the 5B deficient hybrid of UPADHYA and SWAMINATHAN (1963). It would thus appear that there is in fact a mechanism regulating chromosome pairing in T. timopheevii var. zhukovskyi. Clearly an increase in bivalent formation and a concomitant reduction in multivalent frequency must offer some selective advantage to zhukovskyi. Even with the current reduction, the odd-numbered multivalents and asymetric segregation from even-numbered complexes will give rise to aneuploid gametes and zygotes. The somatic chromosome numbers of 7 progeny of the zhukovskyi plant analyzed in Table 2 were distributed as follows; 41-2, 42-4, and 43-1. From the investigations reported in this contribution and from those of other workers, it is apparent that the genomic constitution of T. timopheevii var. zhukovskyi is that anticipated of an autoallohexaploid. Further, there is some evidence of a reduction of multivalent configurations not anticipated from the meiotic data of the hybrid with rye. This, together with other, circumstantial evidence indicates that there may be some genetic regulation of chromosome pairing. Additional investigations have already been undertaken. Literature Cited FELDMAN, M. 1966. The mechanism regulating pairing in Triticum timopheevii. W.I.S. 21 : 1-2. KIMBER, G. and J. LARSEN. 1973. The B-genome of polyploid wheat. Cereal Res. Comm. 1 : 17-26. SHANDS, H. and G. KIMBER. 1973. Reallocation of the genomes of Triticum timopheevii Zhuk. Proc. 4th Int. Wheat Genet. Symp. pp. 101-108. UPADHYA, M.D. and M.S. SWAMINATHAN. 1963. Genome analysis in Triticum zhukovskyi, a new hexaploid wheat. Chromosoma 14.: 589-600. UPADHYA, M.D. and M.S. SWAMINATHAN. 1965. Studies on the origin of Triticum zhukovshyi and on the mechanisms regulating chromosome pairing in Triticum. Indian Jour. Genet. Pl. Breed. 25 : 1-13. (Received October 8, 1975) |
| <-- Back |