Meeting Abstract
Sexually selected traits exhibit remarkable diversity within and among species, much of which can be explained by selection before mating (precopulatory) or selection after mating (postcopulatory). These two episodes of selection interact and may independently influence the evolution of sexually selected traits. Sperm are highly diverse across species, and this diversity has traditionally been thought to be due to postcopulatory selection. To determine if both pre- and postcopulatory selection are associated with diversity in sperm morphology we tested for correlated evolution between testis size, dimorphism and sperm morphology among species of lizards. We collected sperm morphology from 29 species of Anolis lizards that encompass a wide range of both testis size and dimorphism. We measured the length of the sperm head, midpiece, and tail of 15 cells for 5-20 individual males per species. Using phylogenetically controlled methods we tested for correlations between testis size, and sexual size dimorphism and sperm morphology. We found that mean sperm midpiece length was positively correlated with testis size, and that variance in length of the sperm tail was positively correlated with male-biased dimorphism. This suggests that postcopulatory selection selects for larger midpiece size, while precopulatory selection may constrain postcopulatory selection from purging the variation from tail length. We also calculated the Brownian rates of evolution of the head, midpiece, and tail and found that sperm midpiece length evolves 2-3 times faster than other traits in our dataset. This suggests that the midpiece has experienced strong positive selection across species, potentially because it contains the cells mitochondria which are important for sperm function.
