The interaction between X-rays and transposon mobility in Drosophila: hybrid sterility and chromosome loss

Genetic traits associated with P-M hybrid dysgenesis in Drosophila melanogaster were synergistically affected by X-rays. The interaction between damages induced by these two mutator systems was evident when sterility and X/Y chromosome loss were used as endpoints. No interaction was detected in partial chromosome loss, monitored by the loss of BS and y+ markers. The synergism in sterility, measured either as all-or-none or premature sterility (fecundity) was observed when male hybrids derived from different P strains fathers, namely Harwich or II2, were X-irradiated and the effects compared relative to similarly treated non-dysgenic hybrids. Brooding of sperm showed that the effects of ionizing radiation were ionizing radiation were dependent upon the stage of spermatogenesis during which cells were irradiated. The highly synergistic effect on sterility was found when either spermatids or spermatocytes, but not mature sperm, were irradiated with 550 rad of X-rays. These findings were consistent with the higher radiosensitivity of spermatocytes and spermatids to genetic damage and with the correlation between the incidence of sterility and aging of dysgenic hybrids. The latter observation was particularly evident in the case of Harwich P strain-derived male hybrids whose fertility was greatly reduced due to P element mobility. The synergistic effect of X-rays in these dysgenic hybrids resulted in the virtual abolition of the germ line, increasing the sterility from 50% of the untreated 9-10-day old males, to 85% of the treated males when spermatocytes were irradiated. The synergism observed between transposon mobility and ionizing radiation can best the attributed to an interaction between X-ray-induced and P element-induced chromosome breaks. This interpretation is consistent with the more than additive increase in X or Y chromosome loss in irradiated, Harwich P strain-derived hybrid sons. The induction of these events was 1.164% in dysgenic irradiated males as compared to 0.234% in X-irradiated nondysgenic hybrids and 0.40% in dysgenic untreated males. No synergism was observed in X/Y loss in hybrids derived from II2 P strain fathers where the frequency of the events due to P element mobility alone was only one tenth (0.037%) of that found in Harwich-derived hybrids.