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，但与此同时，它们可能在一个物种中积累有害的特征。我们测量了Medaka色素沉着位点的自发生殖细胞突变率，结果约为8×10^<-6>/位点/代，表明脊椎动物物种具有共同的生殖细胞突变率水平。g辐照研究证实，早期分化精原细胞DNA损伤的凋亡对男性生殖细胞整体突变率有重要调控作用。相反，女性生殖细胞的突变率似乎取决于DNA修复的保真度。接下来我们发现了自发染色体断裂。在新泻野生种群中，雌性生殖细胞经常出现缺失。详细分析表明，基因组排列的新机制参与了Medaka野生群体中生殖细胞突变的诱导。此外，我们通过鉴定自发体色突变来分析自发突变的谱。我们对b、ci和i-3突变的相关基因进行了定位克隆，并鉴定出它们是由数个到数十个核苷酸的倒置、重复或缺失引起的。虽然其他报道仅将转座子样元件的插入作为medaka基因（i， el和rs-3）自发突变的原因，但事实证明并非如此。同时，由于我们分离到了3个色素沉着基因（其中2个基因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.