Brain Organization in a Reptile Lacking Sex Chromosomes: Effects of Gonadectomy and Exogenous Testosterone

In mammals, males and females differ both genetically and hormonally, making it difficult to assess the relative contributions of genetic constitution and fetal environment in the process of sexual differentiation. Many reptiles lack sex chromosomes, relying instead on the temperature of incubation to determine sex. In the leopard gecko (Eublepharis macularius), an incubation temperature of 26°C produces all females, whereas 32.5°C results in mostly males. Incubation temperature is the primary determinant of differences both within and between the sexes in growth, physiology, and sociosexual behavior, as well as the volume and metabolic capacity of specific brain nuclei. To determine if incubation temperature organizes the brain directly rather than via gonadal sex hormones, the gonads of male and female leopard geckos from the two incubation temperatures were removed and, in some instances, animals were given exogenous testosterone. In vertebrates with sex chromosomes, the size of sexually dimorphic nuclei are sensitive to hormone levels in adulthood, but in all species studied to date, these changes are restricted to the male. Therefore, after behavior tests, morphometrics of certain limbic and nonlimbic brain areas were determined. Because nervous system tissue depends on oxidative metabolism for energy production and the level of cytochrome oxidase activity is coupled to the functional level of neuronal activity, cytochrome oxidase histochemistry also was performed on the same brains. Hormonal manipulation had little effect on the volume of the preoptic area or ventromedial hypothalamus in geckos from the all-female incubation temperature, but significantly influenced the volumes of these brain areas in males and females from the male-biased incubation temperature. A similar relationship was found for cytochrome oxidase activity of the anterior hypothalamus, amygdala, dorsal ventricular ridge, and septum. The only sex difference observed was found in the ventromedial hypothalamus; males showed no significant changes in cytochrome oxidase activity with hormonal manipulation, but females from both incubation temperatures were affected similarly. The results indicate that incubation temperature organizes the brain directly rather than via hormones arising from its sex-determining function. This is the first demonstration in a vertebrate that factors other than steroid hormones can modify the organization and functional activity of sexually differentiated brain areas.