:Genetic variation in grain-size of rice after irradiation.I.γ-irradiation

Abstract

Dry seeds from a foundation stock of a rice variety Norin No.29, were treated with two doses of gamma rays from 60Co, that is 10, 000 r and 30, 000 r The frequencies of chlorophyll mutations in the X2 progenies from ears having 25 or more seeds were greater with 30. 000 r irradiation than with 10.000 r (Table 2) . Using these materials, mutations which affect quantitatively inherited characters were studied. The polygenic traits chosen were grain-size characters, such as length and width of grain, because it is technically simple and more than one measurement of the character can be made on each individual. With such spatially repeated characters the phenotypic variance can be partitioned into variance within individual and variance between individuals (Table 1). In each X2 or X3 plant five seeds were measured with respect to their length and width. As shown in Tables 3 and 5, the means of all treatments agreed well with each other on both length and width. However, effects of irradiation were remarkable for increased variances between individuals. An increment in variance per unit dose was tabulated in Table 7. The relatively smaller variance increment per unit dose in population treated with 30, 000 r might have resulted from the frequent occurrence of deleterious mutations and/or chromosomal aberrations caused by the high dose of irradiation. The lower rate of induced genetic variation in X3 population, compared with that in X2 populations, seemed to be ascribable to an increase in the frequency of genetic deaths due to the ho-mozygosity of harmful genes in the X3 generation. The calculated coefficients of correlation between length and width of grain in the X2 generation were shown in Table 4. It is very interesting to note that the correlation coefficient for 30, 000 r irradiatin was of the same magnitude as was obtained in the control population. This was ascribed to the lowered viability of plants having a short and broad grain-shape, as was readily seen in Figure 2. Effect of gamma irradiation for that correlation was remarkable in the X3 generation, that is, the correlation coeficient was 0.4000, -0.0020 and 0.1615, respectively for Or, 10, 000 r and 30, 000 r irradiated populations (Table 6 and Figure 4). These results suggest that both grain-size characters are controlled by different neutral genes and the cause of variance increase must be due to the mutations induced in polygenic systems. In order to obtain a maximum amount of genetic variance in quantitative characters, we must irradiate the material with lower doses than when inducing mutations in major genic systems.