Koana, T., Okada, M. O., Ogura, K., Tsujimura, H. and Sakai, K. Reduction of Background Mutations by Low-Dose X Irradiation of Drosophila Spermatocytes at a Low Dose Rate. Radiat. Res. 167, 217–221 (2007).

A sex-linked recessive lethal mutation assay was performed in Drosophila melanogaster using immature spermatocytes and spermatogonia irradiated with X rays at a high or low dose rate. The mutation frequency in the sperm irradiated with a low dose at a low dose rate was significantly lower than that in the sham-irradiated group, whereas irradiation with a high dose resulted in a significant increase in the mutation frequency. It was obvious that the dose–response relationship was not linear, but rather was U-shaped. When mutant germ cells defective in DNA excision repair were used instead of wild-type cells, low-dose irradiation at a low dose rate did not reduce the mutation frequency. These observations suggest that error-free DNA repair functions were activated by low dose of low-dose-rate radiation and that this repaired spontaneous DNA damage rather than the X-ray-induced damage, thus producing a practical threshold.