A brood-pattern analysis of radiation-induced damage in the oocytes of attached-X females of Drosophila hydei was carried out by the dominant lethal test. The methods used allow a refined description of the differential radiosensitivity of oocytes with respect to the stage succession in the oogenesis of this species. As in D. melanogaster, three levels of sensitivity, corresponding to classes A, B and C of oocytes, showed up. The differences in sensitivity led to clear-cut steps in the brood pattern of lethal frequencies, whereby class B is distinguised by an intermediate yield of induced damage.

Eggs deposited on the first day after irradiation are not in every instance a pure sample of irradiated stage-14 oocytes, but may contain and admixture derived from less sensitive vitellogenic stages. For the previtellogenic stages, belonging to class B, it is proposed that it is the youngest group which exhibits the highest radiosensitivity. This agrees with earlier results from detachment of attached-X chromosomes in D. hydei.

The brood patterns of lethal frequencies were not identical, but similar, when a distinction was made between induced early and late embryonic lethality. On certain assumptions, this is interpreted to mean that the stage-dependent differences in sensitivity observed are due to genetic causes, i.e., are correlated with changes in the state of the oocyte nucleus during oogenesis.