Exposure fractionation effects for X-ray-induced dominant lethals in immature (stage-7) oocytes of Drosophila melanogaster: a re-analysis.

Young (0-4-h-old) Drosophila melanogaster females were X-irradiated with single or fractionated exposures over a range up to 6000 R and the induction of dominant lethals in immature (stage-7) oocytes was studied. The results show that (1) the frequencies of dominant lethals are higher after single than after fractionated exposures; (ii) at any given exposure level, the higher the number of fractions, the lower is the frequency of dominant lethals; (iii) consequently, the reduction in dominant lethality relative to single exposures increases with increasing number of fractions; and (iv) this relative reduction in dominant lethality approaches a maximum value when the magnitude of the single X-ray exposure approaches zero (i.e., when the egg survival after single X-ray exposure approaches 100%); the maxima, however, are different for the different fractionation regimes, being higher with increasing number of fractions. These findings are consistent with the assumed kinetics of X-ray induction of dominant lethality in stage-7 oocytes. It is shown that it is possible to predict the expected relative reduction in dominant lethality after fractionation, from appropriate dominant lethal data from single unfractionated exposures.