MECHANISMS AND EVOLUTIONARY CONSEQUENCES OF CRYPTIC FEMALE CHOICE IN DROSOPHILA

Abstract

Sexual selection is proposed to be a powerful driver of reproductive trait diversity, including in traits that act after mating. Extensive work in Drosophila, particularly D. melanogaster, has shown that variation in male postmating traits can be critical for determining reproductive success. However, much less is known about the adaptive significance of female reproductive diversity, especially in postmating traits. In my dissertation, I sought to expand our understanding of how sexual selection shapes postmating trait evolution by studying female postmating adaptations in Drosophila. In Chapter 1, I assessed potential mechanism(s) of cryptic female choice (CFC)—the physical and chemical factors that determine which sperm are used by females after mating—in Drosophila pseudoobscura. Females of this species that occur in sympatry with Drosophila persimilis are known to preferentially select against heterospecific sperm. By assessing matings among multiple populations, I tested three potential mechanisms of CFC: copulation duration, the number of sperm transferred, and female reproductive tract toxicity. I found that reduced copulation duration could play a role in CFC in this system. In Chapter 2, I sought to understand how females make decisions about sperm use. To do so, I assessed whether Drosophila melanogaster females adjust their sperm use in response to male genotype, male courtship effort, male pheromone alteration, and sperm length. Each effect was evaluated factorially across four different D. melanogaster populations. I found that while females generally preferred longer sperm, this choice was dynamic and could actively be altered by female genotype-specific responses to male variation. Finally, in Chapter 3 I assessed the relative contributions of intersexual and intrasexual selection on reproductive trait evolution. By examining behavioral and morphological trait variation across 15 diverse Drosophila species, and complementing these data with up to 15 additional species, I found that male and female postmating traits coevolve, but neither male nor female traits are evolving more rapidly. This result suggests that intersexual selection may be a more important force in driving postmating reproductive trait evolution than intrasexual selection. Together, these findings demonstrate the diverse contributions that female reproductive adaptations can make to postmating sexual selection in Drosophila.