Repair of Dominant Lethal Damage Induced by X-rays in Immature Oocytes of Drosophila Melanogaster

Summary

Immature oocytes (stage 7) of Drosophila melanogaster were exposed to increasing doses of 150 kV x-rays, either continuously or by splitting the dose into two equal fractions separated by a time-interval of 1 hour (dose-rate 850 R/min). Survival was defined as the fraction of hatching eggs, the dominant lethal rate as 1 minus survival. Sigmoid survival curves were obtained both for the continuous and the fractionated treatment. All nine doses investigated yielded higher surviving fractions when applied in a fractionated instead of in a continuous manner. This is explained by repair (at a ‘pre-lethal’ stage) of a fraction F of the dominant lethal damage. F decreases with increasing dose. Dose protraction (e.g. 5 R/min) increases survival still more than dose fractionation does (under the special irradiation conditions of our experiments). A special mechanism of dominant-lethal induction after treatment of immature oocytes of Drosophila melanogaster with x-rays is derived both from the dose-dependence and the dose-rate dependence of survival.