| Research Abstract |
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.
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