Evolution of a cryptic female choice trait is not mediated by postcopulatory sexual selection in Drosophila
Open AccessIn Drosophila melanogaster, males have evolved extremely long sperm due to sperm competition, which occurs within the female’s sperm storage organ, the seminal receptacle (SR). Long SRs select for longer sperm, while shorter sperm have an advantage in shorter SRs. This means that SR length is a mechanism of cryptic female choice (CFC) for sperm length. This male-female interaction drives coevolution between sperm length and SR length across species. While sexual selection theory predicts that CFC drives evolution of the male trait, it is still unclear what drives evolution of the female preference. In this thesis, I ask if post-copulatory sexual selection drives the evolution of the SR as well. Female preference variation is difficult to quantify. However, SR length is easily measurable, allowing us to study evolution of female preference across species. I used female remating rate as a measure of the strength of sexual selection. Higher RT50 corresponds to lower remating rate and a smaller RT50 means a faster remating rate. I obtained SR lengths and female remating rates from the literature and experimental observations and used phylogenetic comparative methods to investigate evolutionary patterns across species. We used linear regression of phylogenetic independent contrasts to test for the evolutionary correlation between SR and remating rate. The results of this comparative phylogenetic analysis corroborated a strong correlation between sperm length and SR length but found no significant association between SR length and remating rate. These results suggest that sexual selection is not driving the evolution of SR length on a macroevolutionary scale. However, previous studies have shown that there is an evolutionary correlation between SR length and sperm length, and that long sperm and long SR genotypes tend to increase fitness for both sexes. These factors may be enough to drive sperm-SR coevolution without selection specifically acting on SR length.
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