Fertility Differences as a Factor in the Maintenance of the "Sex Ratio" Polymorphism in Drosophila pseudoobscura

Abstract

"Sex ratio" (SR) males of Drosophila pseudoobscura produce more than 95% female progeny. Previous workers have had varying opinions on the roles played by differential mating success and fertility of SR and normal (ST) males, if any, in the maintenance of the SR/ST polymorphism. In order to assess the importance of these roles, virgin D. pseudoobscura females were marked and released in the Santa Catalina mountains near Tucson, Arizona. Marked and unmarked females, and males, were collected over a period of about 2 wk, and total numbers of male and female progeny produced by individual flies were counted. The results show that SR males, which appear to produce about half as many sperm as ST males, fertilize as many eggs per insemination as ST males but inseminate only half their share of females. This means that SR males produce as many daughters as ST males, and is consistent with earlier observations that natural populations appear to be in Hardy-Weinberg equilibrium for SR. If the mating disadvantage of SR males is caused directly by reduced sperm production, then SR is not a case of meiotic drive balanced by selection, and the population dynamics of SR in D. pseudoobscura may be representative of other species having SR. If the mating disadvantage is not directly related to reduced sperm production, then SR is a case of meiotic drive balanced by selection. The selection in this case would be the mating disadvantage. If this is the correct explanation then the equilibrium involving SR may be different in every population.