Abstract
To determine whether the linear non-threshold model for stochastic effects of ionizing radiation is applicable to very low dose radiation at a low dose rate, we irradiated immature male germ cells of fruit fly Drosophila melanogaster, with several doses of gamma rays from 60Co, at a dose rate of 22.4 mGy/h. Thereafter, we performed the sex-linked recessive lethal mutation assay by mating the irradiated males with non-irradiated females. The mutation frequency in the group subjected to 500 µGy irradiation was found to be significantly lower than that in the control group (p<0.01), whereas in the group subjected to 10 Gy, it was significantly higher than the control (p<0.03). Several genetic studies suggest that most spontaneous mutations are related to mobile elements, while high dose rate irradiation is known to cause deletions due to restoration of double strand break. Our observation showed that a part of the mutations that should have happened at a spontaneous mutation were not generated apparently. Molecular experiments using microarray indicated that Hsp70, gene for a heat shock protein, was up-regulated after 500 µGy irradiation together with grim, a positive regulator of apoptosis and diver2, a kind of retrotransposon. These results suggested that a stimulation of low dose rate gamma rays might activate frequency of the transposon integrations. However, it is possible that low-dose-rate gamma rays activate cellular apoptosis such that the damaged cells are more efficiently killed compared to non-irradiated cells.
