Pitnick 9806649 Among fruitfly species, the length of sperm and of the female's sperm-storage organs have evolved very rapidly, with some species producing relatively few, gigantic sperm, as much as 20 times their body length. Such unusual sex cells challenge our understanding of the fundamental nature of sex differences and of the evolution of reproductive strategy. Two goals of the proposed work are to determine (1) the evolutionary relationship between these male and female traits and (2) the functional significance of variation in sperm and female sperm-storage organ length. To this end, populations of Drosophila melanogaster will be artificially selected for longer and shorter sperm-storage organs. Comparison between males within the selection lines and in "associated male" lines will permit discrimination of the roles of pleiotropy and sexual selection in generating a correlated response in sperm length to female sperm-storage organ length evolution. Productivity and sperm competition experiments will quantify the relationships between sperm length, female reproductive tract morphology, and patterns of sperm use by females. A third goal of the proposed work is to determine the extent to which divergence in sperm length and female reproductive tract morphology contributes to reproductive failure among populations/species. Factors that prevent interbreeding among populations, or "reproductive isolating mechanisms," are critical to the speciation process. Reciprocal crosses among the selected lines will determine the extent to which divergence in sperm and female reproductive morphology can lead to reproductive isolation among populations. Experiments determining the fate of sperm in hybrid matings among five closely related species will extend this analysis of reproductive isolating mechanisms. Number of sperm transferred, eggs fertilized, and eggs hatching per mating will be determined and used to quantify levels of sperm/female and sperm/egg compatibility for all possible hybrid a nd control crosses. Results from these studies will enhance our presently limited understanding of postmating, prezygotic and postzygotic, extragenomic reproductive isolating mechanisms and broaden our perspective on Drosophila speciation.
