Postmating, Prezygotic Isolation among species of the Drosophila virilis subgroup

Abstract

Reproductive barriers between sexually reproducing organisms prevent interbreeding and gene flow between species. Early studies of reproductive isolation focused on prezygotic and postzygotic isolating mechanisms, yet postmating, prezygotic isolation (PPI) barriers have not been fully explored. In this thesis, I characterized the phenotypic and the evolutionary process of postmating prezygotic isolation among heterosepcific matings in Drosophila. Using species of the Drosophila virilis subgroup and microscopic approaches, I initially examined egg laying, egg hatchability, egg fertilization and sperm storage and retention in D. virilis females’ reproductive tract mated with D. novamexicana males. I found that D. virilis females laid similar numbers of eggs compared to a conspecific mating. However, the number of eggs hatched was significantly lower in heterospecific than conspecific crosses. Furthermore, unhatched eggs were unfertilized. In spite of the large number of sperm transferred to female’s storage, few sperm were retained in storage shortly after mating. I further scored egg laying and hatchability between other heterospecific and conspecific crosses and found that PPI evolved during the diversification of the D. novamexiana – D. americana clade. Finally, eggs laying in heterospecific crosses and the reduction in egg hatchability in heterospecific crosses suggest that females exert cryptic control of the heterospecific ejaculate and influence the process of sperm usage during the fertilization of eggs.