The mus309 mutation, defective in DNA double-strand break repair, increases the frequency of X-ray-induced somatic crossing over in Drosophila melanogaster, but the effect is not dose-rate dependent

Effect of a 1000 R dose of hard X-rays, with two different dose-rates viz. 300 and 1000 R/min on somatic crossing over in the X chromosome of Drosophila melanogaster was studied in two different genotypes. Irradiation was given during the first-instar larval stage of the development. In the control crosses the flies carried wild-type autosomes, but in the experimental crosses the 3 chromosomes carried a DNA double-strand break repair deficient mus309 mutant gene constitution. As expected, the frequency of X-rayinduced somatic crossing over increased in the mutant flies with both dose-rates of irradiation. As also expected, in the control flies irradiation given with the 300 R/min dose-rate caused more somatic crossovers than irradiation given with the 1000 R/min rate. However, rather unexpectedly, in the experimental flies there was no significant difference in the frequency of somatic crossing over between the two dose-rates of irradiation. The results can be explained by assuming that X-ray-induced somatic crossing over is a two-step event, and that the mechanism which repairs the lesion caused by the irradiation is controlled by the mus309 gene. In the control flies the repairing mechanism is capable to recover if the irradiation is given with a short term high dose-rate, but is not capable to recover if the irradiation is given with a long lasting low dose-rate. However, in the experimental mutant flies the repairing mechanism is only poorly recovered irrespective of the doserate.