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 co nstitution and fetal environment in the process of sexual differentiat ion. Many reptiles lack sex chromosomes, relying instead on the temper ature of incubation to determine sex. In the leopard gecko (Eublephari s macularius), an incubation temperature of 26 degrees C produces all females, whereas 32.5 degrees C results in mostly males. Incubation te mperature is the primary determinant of differences both within and be tween the sexes in growth, physiology, and sociosexual behavior, as we ll as the volume and metabolic capacity of specific brain nuclei. To d etermine 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 w ith sex chromosomes, the size of sexually dimorphic nuclei are sensiti ve to hormone levels in adulthood, but in all species studied to date, these changes are restricted to the male. Therefore, after behavior t ests, morphometrics of certain limbic and nonlimbic brain areas were d etermined. Because nervous system tissue depends on oxidative metaboli sm for energy production and the level of cytochrome oxidase activity is coupled to the functional level of neuronal activity, cytochrome ox idase histochemistry also was performed on the same brains. Hormonal m anipulation had little effect on the volume of the preoptic area or ve ntromedial hypothalamus in geckos from the all-female incubation tempe rature, but significantly influenced the volumes of these brain areas in males and females from the male-biased incubation temperature. A si milar relationship was found for cytochrome oxidase activity of the an terior hypothalamus, amygdala, dorsal ventricular ridge, and septum. T he only sex difference observed was found in the ventromedial hypothal amus; males showed no significant changes in cytochrome oxidase activi ty with hormonal manipulation, but females from both incubation temper atures were affected similarly. The results indicate that incubation t emperature organizes the brain directly rather than via hormones arisi ng from its sex-determining function. This is the first demonstration in a vertebrate that factors other than steroid hormones can modify th e organization and functional activity of sexually differentiated brai n areas. (C) 1996 Academic Press.