EXPERIMENTS ON MECHANISMS OF X-RAY INDUCED CHROMOSOME LOSS

The influence of dose fractionation and dose protraction on the induction of X-chromosome loss (leading to viable XO males) after irradiation of mature (stageI4) and immature (stage 7) oocytes of Drosophila melanogaster has been investigated. 150 kV X-rays were used witj a dose rate of 850 R/min or-in the protracted treatments-100 R/min and 50 R/min. In the fractionation experiments the doses were halved and separated by irradiation-free intervals of 20 min or 60 min. Fractionation and protraction of the dose decreases the X-loss frequency when immature, but not when mature oocytes are irradiated. This finding fits the (almost) linear relationship between X-loss frequency and X-ray dose when mature oocytes are irradiated, and the multi-hit curve obtained after treatment of immature oocytes. From the close parallel existing between the radiation response (as regards stage sensitivity, dose dependence and dose-rate dependence) of X-chromosome loss and dominant lethality, common principal causes of these two radiation effects are postulated: on the basis of the experimental data so far available, single chromatid breakage after irradiation of mature oocytes and sister-chromatid breakage after irradiation of immature oocytes are considered as the main mechanisms of both X-chromosome loss and dominant lethality. © 1968.