MULTIPLE MORPHS OF SPERM WERE REQUIRED FOR THE EVOLUTION OF THE SEX-RATIO TRAIT IN DROSOPHILA

In Sex Ratio males of D. subobscura Y sperm degenerate. Nevertheless, Sex Ratio males can produce as many offspring as non-Sex Ratio males. In order to find an explanation for the high number of offspring of Sex Ratio males we analysed spermatogenesis and sperm storage in females. Because D. subobscura has two morphs of sperm, short and long sperm, we studied the relationship of sperm heteromorphism to the Sex Ratio trait in this species. Spermatids of both lengths developed simultaneously in the testes. The first mature sperm produced by a young male were nearly all short sperm. This result indicates that short cysts need less time to mature than long cysts. Thus in a given time more short cysts than long cysts can mature. In the testes of Sex Ratio males more short cysts and fewer long cysts were formed than in the testes of non-Sex Ratio males. Thus in a unit time Sex Ratio males can produce as many or more sperm than non-Sex Ratio males. Females mated to Sex Ratio males had more sperm in their storage organs than those which were mated to non-Sex Ratio males. From the transferred sperm females stored selectively more long than short sperm after mating to a Sex Ratio male than after mating to a non-Sex Ratio male. The effect of the high amount of sperm in Sex Ratio males is twofold: the loss of Y sperm is compensated, and probably in nature a second copulation is prevented by the complete filling of the female storage organs. From these results we conclude that the development and persistence of the Sex Ratio trait in natural populations depends on the presence of sperm heteromorphism.