SICB 2018: Bigger Testes Don’t Produce Bigger Sperm

In the face of mate competition, sperm morphology can vary in a way that can enhance an individual’s chances of siring offspring of females. Studies in the past have attributed increased relative testis size as an approximate measure of an individual’s response to sperm competition. However, this does not take into account the internal architecture of the male testes that may contribute to changes in sperm morphology.

This was the focus of a poster presented by Hanna Hall titled “The evolution of sperm and testis morphology in Anolis lizards” in collaboration with Ariel Kahrl and Michelle Johnson. The authors sampled 2-20 individuals of different species of anoles in Puerto Rico and the Dominican Republic. They compared body size, sperm length ( 15 cells per individual), and the composition and size of various layers of the testis, by conducting a phylogenetic least squares regression on the average values obtained for each species.

The authors found that larger body size was associated with a larger testis size, which was in turn correlated with presence of large seminiferous tubules and a larger luminal area, where mature sperm are stored. Contrary to their expectation though, none of these aspects were associated with producing longer sperm. Further the Gonado-Somatic index (GSI), a common metric that serves as an indicator of relative testis size, was not correlated with any aspects of the internal testis architecture.

An interesting finding in this study was that species with a higher proportion of epithelial cells in the testis produced longer sperm. This result was surprising because larger number of epidermal cells may be associated with smaller spermatogonal cells, which would be predicted to form shorter sperm. The authors suggest that the correlation between lumen area and testis size may result because investment in sperm storage is more important, and that species may be producing large number of sperm which may be longer in length. Nevertheless, more data is needed to understand how changes in sperm morphology affect fertilization success and, further, under what circumstances does size and count of epithelial cells vary. The lack of correspondence of these results with that shown in birds by Lupold et al. 2008 suggests that the mechanisms underlying sperm competition may be taxa or species-specific. We will be eyeing the Johnson Lab for more details on the same in the coming years.