γ-ray-induced mutations of female primordial germ-cells during organogenesis in the silkworm (Bombyx mori L.)

Developing silkworm embryos from 44 up 212 hr after oviposition were irradiated with a constant single dose of 1000 R of 137Cs gamma rays (dose-rate at 300 R per min) and the change of the female primordial germ-cells in the sensitivity to mutation induction during the organogenesis. Egg-color specific locus method was used for the detection of the mutational incidences. The results obtained were summarized as follows: The mutation frequency increased gradually with the development of gonad in embryos from 68 to 116 hr after oviposition and there decreased until 188 hr when the mutation frequency was as low as in the 44 hr old embryo. The frequency increased slightly at 212 hr. The number of germ-cells increased at a rate of 30 hr/generation during the gonad formation in embryos from 68 to 128 hr after oviposition. Gonads were complete in 116 hr-old embryos. The multiplication of primordial germ-cells ceased temporarily after blastokinesis until the onset of gonial divisions in aged embryos 2 days before hatching. The change of primordial germ-cells in radiosensitivity appeared to depend on whether the germ-cells were in the process of multiplication. (author)