The effect of oxygen and nitrogen post-treatments on the survival of irradiated stage-14 oocytes and a possible basis for sensitivity differences between stage-7 and stage-14 oocytes of Drosophila melanogaster

Mature (stage-14) oocytes of Drosophila melanogaster were exposed to different levels of X-irradiation in nitrogen, oxygen or air. Exposures in the first two gases were followed by post-treatments with either nitrogen or oxygen while those in air were followed by nitrogen or air post-treatments. Egg-survival was used as a criterion to study the effects of such treatment combinations. It was found that ( 1 ) the dose-effect relationship observed was consistent with a one-hit survival kinetics, ( 2 ) with post-irradiation anoxia, there was a significant reduction in survival, and ( 3 ) this effect of anoxia could be reproduced even if the flies were post-treated with oxygen for the first 45 min following irradiation, and then with nitrogen for the next 45 min. The available data also support the conclusion that the stage-14 oocytes have more oxygen available than stage-7 cells under normal conditions in air. It is suggested that this differential oxygenation in air between the two germ-cell stages may constitute one of the several factors which contributes to the higher radiosensitivity of the stage-14 cells relative to the stage-7 cells. The Oxygen-Enhancement-Ratio for the stage-14 oocytes has been estimated to be between 3.52 and 3.85, which is much higher than 2.53–2.65 for the stage-7 oocytes.