Temperature effects on lethal mutation rates of Habrobracon oocytes x-irradiated in first meiotic metaphase.

BRIEF note was published in 1940 (WHITING) concerning the influence of A temperature on X-ray sensitivity of different meiotic stages in the Habrobracon oocyte. Unmated females were kept at O"C, 13"C, 25"C, and 35OC for one hour before and one hour after irradiation. During a one half minute of exposure to 212r, they were at room temperature. For oocytes in first meiotic metaphase (metaphase I), mortality was lowest at O"C, highest at 25°C and intermediate at 35°C. The consistent and extensive radiation data on metaphase I obtained since 194.0 made it seem valuable to redesign and to repeat the experiment. When Habrobracon females are X-irradiated, responses of their oocytes in prophase I can be distinguished from those in metaphase I (WHITING 1945). The latter were used in the experiments reported in this paper. LD 50 for the stage is about 400r. The nature of the dose-action curve as well as the absence of any significant differences in response of this curve to different experimental conditions (for example, 124 kv, 50 mev by HEIDENTHAL, C ARK and GOWEN 1955; lOOr/min X-rays, 2OOrJsec cathode rays by HEIDENTHAL 1960; 6.6r/min, SOOr/ min by LACHANCE 1960; and fractionation by WHITING 1945) furnish evidence for one-hit X-ray-induced changes. Tetrad conditions at the time of irradiation and cytological observations on succeeding stages also make the one-hit explanation a plausible one. Viable eggs laid by unmated females develop into haploid males. Those in which dominant or recessive lethal mutations acting on the embryo are induced (as happens in the majority of eggs) fail to hatch.