THE GENETIC SYSTEMS MODIFYING MEIOTIC DRIVE IN DROSOPHILA PARAMELANICA

Abstract

ITHIN the past few years there has been a revival of interest in the phe-wnomenon of non random segregation of chromosomes at meiosis; see for example RHOADES (1952) ; DUNN (1957) ; NOVITSKI (1951) ; SANDLER and NOVITSKI (1957) ; SANDLER, HIRAIZUMI and SANDLER (1958) ; NOVITSKI and SANDLER (1957) ; LINDSLEY and SANDLER (1958) ; SANDLER and HIRAIZUMI (1959) and ZIMMERING (1959). Since non random segregation may result in inequalities in gametic recovery, it is itself a selective force for one allele or chromosome as compared to its opposite member, and this phenomenon, Meiotic Drive, is of importance in any consideration of population genetics and evolution. The first naturally occurring type of non random recovery of meiotic products to be described in Drosophila was the sex-ratio trait in the Drosophila obscura species group (MORGAN, BRIDGES and STURTEVANT 1925; GERSHENSON 1928). Males of the D. obscura group hemizygous for an X chromosome carrying the sex-ratio " gene " may produce predominantly X-bearing sperm and very few that are Y-bearing, thus producing practically all female progeny. STURTEVANT and DOBZHANSKY (1936) have made a cytological study of meiosis in D . pseudo-obscura sex-ratio males and have found that there is loss of the Y chromosome and two equational divisions of the X resulting in no apparent gametic loss and the production of four X-bearing sperm from each primary spermatocyte. In this species the abnormal meiotic behavior showed some irregularity, and it was con-cluded that sex-ratio males should produce, in addition to the many X-bearing sperm, some that were Y-bearing, and also some that were XY or nullo XY. These latter two classes of sperm should then result in production of XXY females