The proportionality between mutation rate and ultraviolet dose after photoreactivation in Drosophila.

1 T has been previously shown that when the polar cap cells of Drosophila (early germ track cells) are treated with various doses of ultraviolet light and the lethal mutation rate plotted against the dose, the resulting dose-rate curve rises rapidly at the lower doses employed. With further increase in dose it becomes less and less steep, and when the dose is large enough to give a detectable rate of from five percent to ten percent, the curve enters a plateau. Finally, it drops somewhat (MULLER et al. 1954). It has also been found (1) that photoreactivating light given as a posttreatment decreases the percent of lethals induced by ultraviolet in the polar cap cells and (2) that the percent decrease is less at the higher doses than at the lower (ALTENBURG and ALTENBURG 1957). Since posttreatment has a relatively smaller effect on the induced lethal rate at high doses than at low, the shape of the dose-rate curve is not the same for posttreated and nonposttreated material. The present studies attempt to determine more precisely the shape of the posttreated dose-rate curve.