Abstract
A threshold was found in the dose-response relationship for Drosophila germ line mutation induced by X-ray irradiation. In the last annual meeting, we have reported that X-ray irradiation induced somatic mutation in Drosophila and that a threshold existed in the dose-response relationship. The threshold in a DNA repair defective strain was smaller than the one in the repair proficient wild type. Most of the results of the sex-linked recessive lethal assays performed in the early 20th century supported the linear non-threshold model. In these studies, however, mature sperms that lacked active DNA repair function were used. Our results suggest that a threshold can exist when the repair proficient immature sperms are used. Here, The sex-linked recessive lethal assay was performed using pre-meiotic young sperms. It was found that (1) mutation was induced in sperms irradiated with 10Gy (dose rate=0.5Gy/min) in a significantly higher frequency than in the non-irradiated control sperms (2) mutation frequency in the sperms irradiated with 10Gy (but dose rate=0.05Gy/min) was the same as control and (3) mutation frequency in the sperms irradiated with 0.2Gy (dose rate=0.05Gy/min) was lower than in control. A threshold was recognized in the dose-response relationship for mutation in immature sperms.
