Evolutionary plasticity in the Pleiotropic regulation of sexually dimorphic traits in gekkotan lizards

Sexually dimorphic traits evolve due to selection for reproductive advantage, and their expression is often functionally correlated by the pleiotropic effects of gonadal steroids (e.g., testosterone, T, in males). When T mediates the correlated expression of traits, then how can changes in individual traits evolve? The main objective of this dissertation was to investigate whether circulating T or androgen receptors in the brain underlie sex- and species-differences in sexually dimorphic trait expression in a comparative study of three species of gekkotan lizards (Goniurosaurus lichtenfelderi, Coleonyx elegans, and Paroedura picta). I hypothesized sex differences in trait expression are due to differences in circulating T, and that the absence of various traits in C. elegans and P. picta are due to a specific alteration in the androgen signaling system rather than a reduction in T. I conducted surgical manipulations to alter levels of T in adult males and females of each species. Testosterone-sensitive traits included courtship, copulatory mounting, and aggressive behaviors, secretions from precloacal pores, enlargement of the hemipenes and head width. Elevated T in males was almost always the primary mediator of sex differences in trait expression within species. Some, but not all, of the male-typical traits were induced in adult females, suggesting the neural or physiological substrates underlying certain traits are permanently differentiated between the sexes prior to adulthood. Traits absent from the phenotype of a species, such as courtship in C. elegans and P. picta, cannot be induced by exogenous T. I hypothesized differences in behavioral sensitivity to T would be due to differences in androgen receptors in brain regions associated with control of reproductive behaviors. Immunohistochemistry revealed P. picta had increased abundance of androgen receptor immunoreactivity (AR-ir) in the preoptic area and ventromedial hypothalamus relative to G. lichtenfelderi. Thus, the abundance of AR-ir does not reflect the expression of courtship and aggressive behaviors in these species. Although circulating T or AR-ir do not explain interspecific differences in trait expression, results indicate correlated traits mediated by the pleiotropic effects of T are not constrained but targeted changes in sensitivity to T allows evolutionary plasticity in trait expression.Ph. D.Includes bibliographical referencesby Alison Golinski Goldber