Investigations on radiosensitive and radioresistant populations of Drosophila melanogaster. V. Relations between the relative radioresistance and some recombination properties in the irradiated population ROI

Experiments were conducted to inquire whether the radioressitance observed in an irradiated laboratory population (RÖI) of Drosophila melanogaster might be associated in some way with recombinational processes. Simultaneously, data were collected on the stage distribution of radioresistance in RÖI by studying the induction of dominant lethals and X-chromosome losses in mature females at various exposure levels of X-irradiation (in eggs sampled from subsequent 12-h broods).The data show that (1) the radiation response of both populations (RÖI and its control + K) is equal in the highly sensitive mature stages, (2) RÖI is resistant relative to +K in the medium-sensitive stage-7 and younger oocytes collected on days 1.0 to 5.5 after exposure, and (3) the difference between the populations disappears again when the sensitivity steeply decreases on days 5.5 to 6.5. Similar brood-pattern experiments indicate that exchanges between homologous chromosomes are induced (by temperature shock or X-irradiation) in eggs sampled after day 5.5. Thus it is evident that the relative radioresistance in RÖI is due to mechanisms which operate in the developing oocyte in the stages of a medium radiosensitivity between that phase in which recombination is inducible and stage-14.The observed temporal sequence of recombination and relative radioresistance in RÖI supports the speculation that the latter might be associated with recombination repair. However, the natural recombination frequencies were equal in +K and RÖI. Likewise, no clear evidence was obtained on differences between the two populations with respect to X-ray-induced modifications of homologous exchanges in various para- and pericentric parts of the genome.