| Results and Discussion The utility of monosomics and nullisomics in cytogenetic studies depends upon their breeding behaviour. Theoretically, one expects an individual with 2n-1 chromosomes to produce 'n' and 'n-1' gametes in equal frequency. Like wise one should be able to obtain '2n' and '2n-1' individuals in equal frequency from (2n-1) x 2n crosses. Further more, 2n-1 individuals when selfed should yield 2n, 2n-1 and 2n-2 progeny in the ratio of 1:2:1. However, this is never realized in many cases and the departure from this expected ratio is indeed varied. The causes for this departure from expectation are: 1) production of 'n' and 'n-1' spores in unequal frequency; 2) reduced viability of 'n-1' spores; 3) competition between 'n' and n-1' microspores; 4) reduced viability of '2n-1' zygotes; and 5) reduced viability of nullisomic zygotes. These factors mainly determine the breeding behaviour of the monosomics. The female parent, i.e. monosomic line Pb C 591 may produce either 'n' or 'n-1' gametes in different proportions. It is known that the transmission of monosomic condition from female side is 75 percent. Thus when monosomic lines of Pb C 591 were used as female parent and the tetraploid wheat variety Bijaga Yellow as male parent 75 percent of monopentaploid plants were expected in F1 generation. However, on an average 59.44 percent of the plants were monopentaploids (13"+8'), 36.54 percent eupentaploids and in the remaining 4.02 percent plants, the chromosome number was 2n=33 (Table 1). The occurrence of monopentaploids was maximum in line 5A, and in line 2A it was minimum. In first back cross generation (BC1) plants with chromosome configurations from 13"+2' to 13'+8' occured (Table 2). But plants with 13"+1, were observed, which agrees with, MOCHIZUKI (1968b). In the BC1 generation, about 9.90 percent of the populations had 13"+2', 18.80 percent had 14", 14.36 percent had 13"+5', 10.40 percent had 13"+6', 2.48 percent had 13"+7' and 15.25 percent had 13"+8'. The frequency of plants with 14 bivalents in the BC1 generation, suggested that increasing homozygosity of the tetraploid complement in monopentaploid wheat hybrid reduced the frequency of univalent chromosome transmission. The low frequency of 13"+2' differed from MOCHIZUKI (1968b) who obtained about 30 to 50 percent plants with 13"+2'. |
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