Dose-rate effect on transgenerational mutation frequencies in spermatogonial stem cells of the medaka fish.

The estimation of transgenerational genetic risk of radiation exposure to non-human species is crucial for the protection of ecosystems. Here we determined the frequency of specific-locus mutations at the five pigmentation loci in medaka spermatogonial stem cells after gamma irradiation at 0.03 cGy/min and 95 cGy/min. At each total dose, the mutation frequency was significantly lower in the 0.03-cGy/min group than in the 95-cGy/min group, suggesting a dose-rate effect. The ratio of the induced mutation frequency at 0.03 cGy/min to that at 95 cGy/min was approximately 0.42 from 0 to 1.9 Gy and approximately 0.33 from 1.9 to 4.75 cGy. In the mouse, this ratio is estimated to be 0.33 (Russell and Kelly, Proc. Natl. Acad. Sci. USA 79, 542-544, 1982). It is thus possible that the magnitude of the dose-rate effect on transgenerational mutation frequencies is comparable between mouse and medaka spermatogonia, suggesting similar dose-rate effects among vertebrates.