High incidence of mosaic mutations induced by irradiating paternal germ cells of the medaka fish, Oryzias latipes.

Delayed-type mutations induced by radiation have recently been demonstrated in various somatic-cell systems. Such mutations are thought to result from the transmission of genetic instability through many cell divisions subsequent to a single exposure to ionizing radiation. Here, we have examined whether 'transgenerational' delayed-type mutations can arise during embryonic development of the medaka fish as a result of exposing the sperm and spermatids of live fish to 137Cs gamma-radiation. To do this, we made use of a sensitive specific-locus test (SLT) for the medaka that we have recently developed. Because the medaka has a transparent egg membrane and embryo body, both visible mosaics and whole-body mutations can be detected during development at an early-expressed pigmentation locus. When wild-type +/+ males were gamma-irradiated and then mated with wl/wl females, the frequency of F1 embryos with both wild-type orange leucophores (wl/+) and mutant-type white leucophores (wl/wl*) (mosaic mutants) was about 5.7x10(-3)/Gy. The frequency of embryos with only white leucophores (whole-body mutants) was about 1.3x10(-3)/Gy. These results suggest that delayed mutations frequently arise in medaka fish embryos that have been fertilized with irradiated sperm. Some possible mechanisms involved in the generation of these delayed mutational events (including genomic instability in the early embryos) are discussed.