The evolution of testis architecture and sperm size in <i>Anolis</i> lizards

Abstract The major function of the testes is the production of sperm cells. Across species there are dramatic differences in sperm morphology, and this variation may result from variation in the internal structure of the testis. Yet, the extent to which testis anatomy is associated with sperm size remains unclear. In this study, we used 20 species of Anolis lizards to examine interspecific relationships between testis architecture and sperm component lengths, as well as testis architecture and a proxy for copulation rate (hemipenis muscle damage), using both phylogenetic and regular linear models. Results of our evolutionary analysis indicate that testis and sperm components have high phylogenetic signal. Further, testis architecture and sperm component length, and the variance in these traits, are not associated with one another using phylogenetic linear models, although non-phylogenetic analyses suggest that larger testes produce sperm with longer flagella. Our interpretation of these conflicting models is that the strong phylogenetic signal of these traits may mask significant functional and developmental relationships in phylogenetic models that regular linear models reveal. This suggests that these traits are not experiencing selection that would cause these traits to covary in a different pattern than their phylogenetic relationships; instead, in this group they are likely evolving together neutrally.