A novel environment-induced genic male sterile (EGMS) mutant in indica rice
SummaryMale-sterile mutants were isolated from M2 and M3 generations of indica rice variety 26 Zhaizao, dry seeds of which had been exposed to 60Co-γ rays at a dose of 290 Gy. The mutants were planted in early season and ratooned in late season for two successive years for identification of fertility conversion in different growing seasons. One of the mutant lines was further observed in a growth chamber and in the field. Results showed that daily average temperature might be the major factor conditioning the male fertility conversion at a moderate daylength. The critical temperature for the male fertility conversion of the mutant grown under 12.5 h and 14.0 h daylength is about 23°C, below which the plant becomes completely male sterile. Its male fertility conversion character differs from other EGMS lines so far developed. The performance of the hybrids between the mutant and some other indica varieties demonstrated its good combining ability and its potential value in hybrid rice production. The obtained mutant line still sheds KI-stainable pollen grains under male sterilizing conditions. Nevertheless, pollen grains shed from the male sterile plants were much more vulnerable than from normal plants. At sucrose concentration below 1.5 M, the pollen grains from the mutant grown under male sterilizing conditions almost completely broke down, while above 1.5 M they became plasmolysed and shrunken. This is indicative of poor development of the membrane and walls of the pollen grains from the male sterile mutant, causing the pollen grains to be unfunctional. NBT test also clarified the abortion of the pollen grains from the mutant, which were formed in the male sterilizing environment.
Genetic analysis of male sterility in rice mutants with environmentally influenced levels of fertility
SummaryTwenty-six male sterile plants grown in the field were recovered in the M7 generation from ethyl methane sulfonate-treated material of the rice cultivar M-201. Fertility increased five-fold when ratooned plants from the field were grown in a growth chamber with a 12 hour daylength. Crosses between mutant and normal fertile cultivars produced fertile F1 plants. Female fertility was normal as judged by percent seed set from unbagged panicles of parental and recombinant lines. Transgressive segregation for fertility was observed for all crosses in the F2 and F3 generations. Five of 37 F3 male sterile plants showed moderate levels of seed fertility under winter greenhouse conditions and reduced seed set when transplanted to summer field plots. Fertility data from reciprocal crosses suggested cytoplasmic factors had little or no effect on levels of male sterility in the mutant lines. Chi-squared analyses of F2 and F3 generation results indicated male sterility of the mutants is conditioned by two nuclear genes with epistatic effects.
Recurrent Selection by Using Genetic Male Sterility for Rice Improvement
Rice breeders often encounter the situation in which it is very difficult to recombine desirable agronomic traits of different genetic sources and there is no room to choose other sources. These difficulties are supposedly arisen from genetic linkage or pleiotropy. However, as far as linkage is concerned, it is highly possible to break it up by some breeding techniques. Recurrent selection seems to be a very effective breeding technique to dissipate undesirable linkage and to increase desirable gene combinations in a population. This technique, however, has been exclusively used upon crosspollinated crops. There is no reason to exclude its use upon self-pollinated crops. The principal handicap in using recurrent selection upon self-fertilized crops like rice is the difficulty to make random crossing for recombination in each recurrent cycle. The present paper proposes a new breeding technique, that is, recurrent selection using a male sterile factor, to overcome the difficulty imposed by undesirable genetic linkage for rice improvement.