Induced Changes in Female Germ Cells of Drosophila. VII. Exchanges Induced in Different X Chromosome Regions after X-Raying Oocytes and Oogonia.

OME of the earlier papers in this series (HERSKOWITZ and ABRAHAMSON 1957; S H ~ ~ ~ ~ ~ ~ ~ ~ ~ 1959) were concerned with the detection of X-ray-induced exchanges in germ cells of female Drosophila melanogaster. It was found in the proximal portion of the X chromosome, i.e., the region between the right end of the chromosome and the locus of carnation, that X-ray exposure induced exchanges both in oogonia and in oocytes. In the next proximal region of the X chromosome (between carnation and forked) there was also an increase in the rate of exchanges recovered after X-raying oogonia and probably also young oocytes; however, there was no such increase after more mature oocytes were X-rayed (HERSKOWITZ 1959). In order to more fully understand the nature and distribution of the exchanges induced by X rays, it seemed desirable to investigate further the exchange responses of both proximal and distal chromosome regions. Accordingly, the present experiments were designed to detect X-rayinduced exchanges in four regions of the X chromosome-two of them proximal (the carnation-right end of chromosome segment was divided into two regions) and two distal (yellow-white, and white-split). The results are analyzed in an attempt to determine the genetic nature and the quantities of the exchanges induced in these different regions of the X at different stages in oogenesis. The bearing these results have on the mechanism of origin of X-ray-induced recessive lethal mutants in female germ cells is discussed briefly.