Sex-ratio trait in Drosophila pseudoobscura – Fertility relations of males and meiotic drive.

In the early analysis of the “sex-ratio” polymorphism (SR) of Drosophila pseudoobscura, complete meiotic drive was assumed, and study centered on the nature of the selective forces opposing its spread. Policansky and Ellison (1970) found that the mechanism of SR involved the degeneration of half the spermatids during spermatogenesis. They suggested that little or no drive may be expressed due to a fertility deficiency of SR males. Thus little or no selection is required to balance the drive. I report studies conducted to determine the conditions under which either of these alternatives might hold. Virgin females were mated once to either SR or ST (standard) males aged as virgins for 3 days, and their fecundities were determined by daily egg counts. No differences were found between the two groups in either daily egg production or egg-to-adult survival of the progenies. Only females maintained with males throughout the experiment were clearly superior in these parameters. No fertility differences between SR and ST males are unconditional. Fertilities of males aged for varying lengths of time as virgins were determined by mating them to as many virgin females as they would inseminate in a brief span of time. Time periods ranged from 3 to 8 h, depending on the ages of the males. Fertility of the SR males was lower than that of ST males of the same age for males aged 20-33 h from eclosion. Differences in 4-day-old males were not so clear. When 4-day-old males which had depleted their stores of sperm by repeated matings were retested after 12 h of rest, SR males again showed lower fertility than ST males. The fertility reduction of SR males occurs only during the first few days as adults or after repeated matings. The effect of different degrees of insemination on the pattern of production of fertile eggs was examined by mating virgin females to males which were either virgin (heavy insemination) or with seminal vesicles partially depleted by three previous matings (light insemination). No differences in either fecundity or hatchability between the groups were observed early in the reproductive period, but the hatchability of the eggs produced by the lightly inseminated females declined sooner than that of the heavily inseminated group. This response would minimize the populational consequences of fertility differences in males unless the females remate. It is suggested that the conditional fertility differences between SR and ST males are of little consequence until the females remate. Depending on the time of remating, there is potentially a wide range in the amount of meiotic drive expressed.