Evolutionary changes in dopaminergic modulation of courtship behavior in Cnemidophorus whiptail lizards

Preoptic dopamine release is integral to the display of copulatory behaviors in male mammals and birds. However, while the anatomical distributions of the dopamine synthesizing enzyme tyrosine hydroxylase are similar among vertebrates, evolutionary changes in the functional role of dopamine are poorly understood. In this study, we tested whether a dopamine D1 receptor agonist would facilitate the display of courtship and copulatory behaviors in two related Cnemidophorine lizards (Cnemidophorus inornatus and Cnemidophorus uniparens). Cnemidophorus lizards offer a unique system to study evolutionary changes in functionality because ancestral (e.g., C. inornatus) and descendant (e.g., C. uniparens) species can be studied in parallel. Cnemidophorus uniparens is an all-female, parthenogenetic species and is the triploid descendant of the sexual and diploid species C. inornatus. Here we report that in castrated male C. inornatus and ovariectomized C. uniparens a dopamine D1 agonist increased the proportion of individuals mounting and decreased the latency to mount. Moreover, there was a species difference in sensitivity to the agonist: Mounting was elicited at a lower dose in C. uniparens than in C. inornatus. One possible explanation for this heightened sensitivity in the triploid parthenogen is that, by virtue of the increased ploidy, the parthenogen has elevated levels of D1 receptor in limbic brain areas modulating courtship behavior.