SPECIES-DIFFERENCES IN ESTROGEN-RECEPTOR AND PROGESTERONE RECEPTOR-MESSENGER-RNA EXPRESSION IN THE BRAIN OF SEXUAL AND UNISEXUAL WHIPTAIL LIZARDS

Circulating concentrations of gonadal steroid hormones and reproductiv e behavior in female vertebrates vary as a function of ovarian state. Steroids secreted by the ovary, specifically estrogen and progesterone , influence the expression of behaviors associated with reproduction b y interacting with intracellular sex steroid receptors located in spec ific regions of the brain. Using in situ hybridization, we analyzed es trogen receptor and progesterone receptor messenger RNA expression in several brain regions of ovariectomized, vitellogenic, and postovulato ry individuals from two species of whiptail lizards (Cnemidophorus uni parens and C. inomatus). Although these species are genetically very s imilar, they differ in two aspects of their reproductive biology: (i) the unisexual C. uniparens alternate between expressing female-typical and male-like pseudosexual behaviors while female C. inomatus normall y express only female receptive behavior, and (ii) circulating estradi ol concentrations in reproductively active female C. uniparens are app roximately five-fold lower than in reproductively active female C. ino matus. We found that the regulation of sex steroid receptor gene expre ssion was region specific, with receptor-mRNA expression being increas ed, unchanged, or decreased during vitellogenesis depending on the are a. Furthermore, several species differences in the amount of sex stero id receptor-mRNA were found that may be relevant to the species differ ences in circulating estrogen concentrations and sexual behavior.