Cytogenetical analysis for the action of a mutator in rice

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A mutator and a recessive slender-glume(SG) gene were simultaneously induced by gamma-ray irradiation of seeds of a rice variety Gimbozu (GB). The mutator is activated by a reverse mutation of SG gene to normal-glume gene, which frequently occurs in both somatic and gametic cells, inducing various types of mutations in many loci of quantitative and qualitative characters. To clarify the process of mutator activation, SG gene and its reverse mutation were studied for their effects on somatic cell division and meiotic chromosome behavior.Mitotic index(MI) reached the maximum value at 66 hours after sowing in SG lines, while less than 54 hours in GB. Since the time from sowing to the maximum MI reflects the time to the first anaphase from sowing, the delay more than 12 hours observed in SG line suggests that SG gene prolongs cell-cycle time. SG gene reduced the root length at germinating stage, seedling height and culm length, and delayed heading date. These growth inhibitions seem to have resulted from the prolonged cell-cycle time.Cells with a pair of univalent chromosomes (11II+2I cells) were observed at the diakinesis of pollen mother cells in SG plants and in F_1 plants from reciprocal crosses between SG plants and GB, whereas not observed in GB. The frequency of 11II+2I cells differed among three SG lines which were different from one another in the reverse mutation rate, varying in parallel with the reverse mutation rate of line. This strongly suggests that the appearance of non-pairing homologous chromosomes is the cause or the effect of the reverse mutation of SG gene.