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Sexual Selection is not a Driver of Female Sperm Storage Organ Length in Drosophila


Authors:
Cameron Himes; Tiffini Smith; Mollie Manier

Affiliation: The George Washington University

Keywords:
h. selection; g. phylogenetics

In sexual selection, male trait evolution is often driven by female preference in a way that can generate exaggerated male phenotypes via a Fisherian runaway process. In this way, character transitions in the male trait are expected to follow female preference transitions. However, factors driving the evolution of female preference are less understood. Sperm in Drosophila are extremely long and are stored within the long, coiled female sperm storage organ, the seminal receptacle, or SR. When females mate with multiple males, long SRs favor long sperm, and short SRs favor short sperm. Thus, sperm length and SR length are a post-copulatory male trait-female preference system. Consistent with this, sperm length and SR length are coevolving across the Drosophila lineage, driven by a genetic correlation between the two traits. However, it is unknown what drives SR evolution. To test the hypothesis that SR length evolution is correlated with post-copulatory sexual selection, we obtained remating rates, SR length, sperm length, body size, and fecundity in 81 Drosophila species through direct experimentation and conducted a comprehensive literature search. We performed a comparative phylogenetic trait evolution test to determine if there is correlated evolution between remating rate and SR length in a phylogenetic context. There was no significant correlation between remating rate and SR length, suggesting that sexual selection is not driving the evolution of SR length on a macroevolutionary scale. The genetic correlation between SR and sperm lengths and their respective competitive phenotypes may be enough to drive sperm-SR coevolution without selection specifically acting on SR length.