Elucidation of mechanism for fixation of mutations occurred in germ cells using medaka.

在生物体中引起的生殖细胞突变是进化所需的遗传变异的来源，但同时，它们可能会在一个物种中积累有害的特征。我们测量了Medaka在色素基因座上的自发生殖细胞突变率，得到的值约为8×10~(-6)/基因座/世代，表明脊椎动物具有共同的生殖细胞突变率水平。研究表明，DNA损伤的早期分化精原细胞的凋亡对男性生殖细胞的总体突变率具有重要的调节作用。相比之下，女性生殖细胞的突变率似乎取决于DNA修复的保真度。接下来我们发现自发的染色体断裂。而且在新泻野生种群的雌性生殖细胞中经常会导致缺失。详细的分析表明，一种新的基因组排列机制参与了青竹野生种群生殖细胞突变的诱导。此外，我们通过鉴定自发的体色突变来分析自发突变的谱。我们对b、ci和i-3突变体的相关基因进行了定位克隆，并鉴定了它们是由几个到几十个核苷酸的倒置、复制或缺失引起的。尽管其他人只报道了转座子样元件的插入是青竹基因(i、el和rs-3)自发突变的原因，但事实证明情况并非如此。同时，由于我们克隆了三个色素沉着基因(其中两个的功能是本研究新发现的)，我们可以为色素细胞的发育和进化研究提供新的见解。

Germ-cell mutations caused in an organism are the source of genetic variation that is required for evolution, but at the same time, they may accumulate deleterious traits within a species. We have measured spontaneous germ-cell mutation rate at the pigmentation loci of the Medaka The resulting value was about 8×10^<-6>/locus/generation, indicating that vertebrate species share a common level of germ-cell mutation rate. Based on g-irradiation study, apoptosis of early differentiating spermatogonia with DNA damage proved to have an important role for regulation of overall mutation rate in male germ cells. In contrast mutation rate in female germ cells appeared to depend on fidelity of DNA repair. Next we found that spontaneous chromosomal breakages. and deletions are frequently caused in female germ cells of the Niigata wild population. Detailed analyses suggested that a novel mechanism for genome arrangement is involved in induction of germ-cell mutations in the wild populations of the Medaka Furthermore, we analysed the spectrum of spontaneous mutations by identifying spontaneous body-colour mutants. We positionally cloned responsible genes for the b, ci and i-3 mutants and identified they are caused by inversion, duplication or deletion of from several to dozens of nucleotides. Though others reported only insertions of transposon-like elements as a cause of spontaneous mutations on medaka genes (i, el, and rs-3), it is proved that that is not only the case. Meanwhile, because we isolated three pigmentation genes (functions of two of them are newly revealed by this study), we could provide new insights for developmental and evolutional studies of pigment cells.