EFFECT OF X-RAYS AND HIGH TEMPERATURE ON CROSSING OVER

Abstract

I n the course of comparative studies of the effect of x-rays and of high temperature on the crossing over frequency in the centromere region of the I I I chromosome in the females of D. melanogaster these two factors have been observed to increase the cross-over value in different broods derived from the treated females. The effect of high temperature proved to be limited only to the broods of 4—6 days after treatment. On the contrary, x-rays have been observed to increase the cross-over value in many successive broods. This suggests that high temperature is a factor, the effect of which is specific for but one stage of oogenesis, presumably the moment of chromosome pairing, while x-rays induce crossing over being applied not only during the prophase of the first meiotic division, but also at the preceding stages of oogenesis. Some changes of chromosomes of oogonia that result in chromosomal exchanges later, at the prophase of the first meiotic division, might be supposed to be induced by x-rays. It is suggested by the results of the studies in the effect of the successive application of x-rays and high temperature that these two factors act independently.