Meiotic X-y Exchange and Nondisjunction Induced by Irradiation in the Drosophila Male.

BSERVATIONS on X-irradiation induced X-Y nondisjunction and exchange in spermatocytes of Drosophila have been made by STRANGIO (1961, 1962) and by ZIMMERING and Wu ( 1963,1964), both using the doubly marked Y chromosome, BS Y y+ (BROSSEAU, NICOLETTI, GRELL, and LINDSLEY 1961), and by SAVHAGEN (1961) who employed the singly marked Y chromosome, y+ Y (= scs. Y of MULLER 1948). The criterion for nondisjunction was the recovery of F, females carrying, in addition to the X from the parental female, an X and Y derived from the irradiated male; that for exchange was the recovery of F, females possessing, in addition to the X derived from the parental female, an X chromosome bearing a Y-derived marker beyond its centromere which came from the X-rayed male. While STRANGIO reported a higher frequency of nondisjunction than exchange, the more critical genetic tests of ZIMMERING and Wu showed that the reverse situation actually obtains. SAVHAGEN reported results similar to those of STRANGIO. However, as suggested by ZIMMERING and Wu (1964), SAVHAGEN’S genetic test for distinguishing nondisjunction and exchange may have been inadequate, leading to misclassification of cases of exchange as nondisjunction but not the reverse. It was with this possibility in mind that the present experiment with the y+Y was carried out. The results are clearly in accord with the previous findings of ZIMMERING and Wu in demonstrating a considerable excess of X-ray induced exchange over nondisjunction.