INFLUENCE OF DOSE AND GERM-CELL STAGE ON X-RAY-INDUCED CROSSOVERS IN MALE DROSOPHILA

Newly hatched Drosophila males heterozygous for b pr and vg were exposed to 1000 rad and mated daily to fresh b pr vg females. The first crossovers appeared on day 6 (1.50/00). On days 7, 8 and 9 there was a peak incidence of crossovers (between 6 and 100/00). We conclude that these were induced in spermatocytes. There was a decline in incidence over days 10 and 11, with increasing evidence of clustering, to day 12 (20/00) with maximum clustering. Evidence is presented for concluding that days 10 and 11 represent irradiated secondary spermatogonia and day 12 and later, stem cells. Crossovers induced in spermatocytes have lower fertility and viability than those induced in spermatogonia, but this effect is largely due to effects of irradiation on spermatocytes which have been induced independently of the crossover event. After 3000 rad the peak of crossing-over (3%) is obtained on days 10 and 11 (secondary spermatogonia). By studying the induction of crossovers in concurrent experiments at 1, 2, 4 and 6 krad and using different markers (dp b cn bw) we have confirmed that the sensitivity pattern is dose-dependent. The explanation for this appears to be that whereas the yield of crossovers in spermatocytes and stem cells is linearly dependent on dose, the induction of crossovers in determinate spermatogonia is proportional to the dose to the power of 3 2. © 1968.