Effect of X-irradiation on the size of the mutated sector in diploid and tetraploid rice.

An investigation was carried out of the effect of X-irradiation on the size of the mutated sector in three diploids (A.S.D.8, T.N. 1 and G.E.B. 24) and the autotetraploid of G.E.B 24 of Oryza sativa L., by analyzing the M2 segregation ratios of chlorophyll mutants. A total of 612 segregating M1 panicle progenies in the diploids and 284 panicle progenies in the tetraploid was studied in the M2 generation for the ratio of normal to chlorophyll mutant seedlings. Of the 573 segregating M1 panicles analysed by chi square test in the diploids, 206 showed good fit with a 3∶1 ratio, while the rest of the progenies (367) deviated significantly in both directions from the expected 3∶1 segregation. This was interpreted as the result of M1 panicles developing from both single cell and multicellular initials following X-irradiation, the former segregating for 3∶1, the latter significantly deviating from 3∶1. An assessment of fertility (as judged from the number of M2 seedlings per mutated M1 panicle), related to the distribution of chi square deviations from the 3∶1 ratio in the diploids, indicated that the M1 panicle progenies deriving from single cell initials had low fertility but those originating from multicellular initials ranged from low to high fertility. A comparison of the distribution pattern of the M2 segregation ratios of chlorophyll mutant phenotypes in the diploid and autotetraploid G.E.B. 24 indicated that, in the tetraploid, a larger sector and only one, or fewer cell initials than in the diploid species, are involved.