The kinetics of X-ray-induced somatic crossing-over in Drosophila melanogaster and the effects of dose fractionation

Nonlinear dose-response curves have been obtained with the X-ray induction of somatic crossover spots on Drosophila tergites, possibly reflecting the formation of independent breaks in two homologous chromatids in G₂ with restitution by crossing-over. Interrupting the irradiation dose for some time reduces the number of crossover spots observed compared with that found on flies irradiated continuously with the same total energy. The effect of dose fractionation is to separate the two breaks in time so that the pair of ends at the first break in the chromatid are rejoined to their original configuration before a new pair of ends is available for interaction. The amount of repair observed with 10 min separation is comparable to that observed from stage-7 oocytes.