The response of the immature testis of Drosophila to the mutagenic action of X-rays

Summary Drosophila melanogaster males were irradiated at various stages of pre-imaginal development, and mutation rates were compared between first broods of different age groups, and between successive broods in the same age group. The findings were correlated with observations on germ cell development in the larval and pupal testis. Sensitivity to the mutagenic effect of X-rays is low in all premeiotic stages. It increases suddenly and dramatically with the onset of meiosis, remains stationary through meiosis, and subsequently increases again up to a peak in the late spermatid stage when transformation into morphologically mature spermatozoa is taking place. This stage provides sperm for the first brood from males treated at about the middle of the pupal period, and mutation rate for a given dose of X-rays is here about 4 times as high as in mature sperm and about 1 1/2 times as high as during meiosis. Subsequently, mutation rate drops again gradually to the level characteristic for mature sperm. Germinal selection cannot account for any of these changes; but it may be the main cause for the slight increase in response which occurs during spermatogonial development.With 10 figures in the text.