Cytogenetical analysis for the action of a mutator in rice

利用伽玛射线辐照，同时诱变水稻品种金伯祖（GB）的一个突变基因和一个隐性细长颖片（SG）基因。该突变子是由SG基因向正常颖质基因的反向突变激活的，这种突变在体细胞和配子细胞中都经常发生，在许多数量和质量性状的位点上诱导各种类型的突变。为了阐明突变子的激活过程，研究了SG基因及其反向突变对体细胞分裂和减数分裂染色体行为的影响。SG系有丝分裂指数（MI）在播种后66 h达到最大值，而GB系则在54 h以下。由于从播种到最大MI的时间反映了从播种到第一个后期的时间，因此在SG系中观察到的延迟超过12小时说明SG基因延长了细胞周期时间。SG基因降低了发芽期根长、苗高和茎长，推迟了抽穗日期。这些生长抑制似乎是由于细胞周期时间的延长。在SG植株和g与GB正交的F_1植株的花粉母细胞发育过程中发现一对单价染色体（11II+2I细胞），而在GB植株中未发现。3个SG系的11II+2I细胞的频率不同，且不同的SG系的反向突变率不同，其变化与系的反向突变率平行。这强烈提示非配对同源染色体的出现是SG基因反向突变的原因或结果。

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.