INDEPENDENT EFFECTS OF INCUBATION-TEMPERATURE AND GONADAL SEX ON THE VOLUME AND METABOLIC CAPACITY OF BRAIN NUCLEI IN THE LEOPARD GECKO (EUBLEPHARIS-MACULARIUS), A LIZARD WITH TEMPERATURE-DEPENDENT SEX DETERMINATION

The extent to which variation within and between the sexes can be assi gned to genes vs. environment is problematic, because, in most vertebr ates, males and females differ genetically. However, factors other tha n sex chromosomes and the consequent sex-typical gonadal hormone secre tions may play important roles in the differentiation of the neural me chanisms underlying individual and sex differences in aggressive and s exual behavior. The leopard gecko, like many oviparous reptiles, lacks sex chromosomes. Instead, gonadal sex is determined by temperature du ring embryogenesis, with low and high incubation temperatures producin g females and intermediate temperatures producing mixed sex ratios. In essence, this allows for the study of individual and sex differences without the confounding variable of genetically determined gender. Exp eriments have shown that the temperature experienced during incubation plays a critical role in establishing the adult morphological, endocr inological, and behavioral phenotype. In this experiment, the independ ent effects of incubation temperature and gonadal sex on the morpholog y and metabolic capacity of specific brain nuclei were determined. Bot h individual and sex differences in the volume of the preoptic area an d ventromedial nucleus of the hypothalamus are determined primarily by incubation temperature, not by gonadal sex. However, incubation tempe rature and gonadal sex are both important in determining the metabolic capacity in the anterior hypothalamus, external amygdala, dorsal late ral nucleus of the hypothalamus, dorsal lateral nucleus of the thalamu s, dorsal ventricular ridge, habenula, lateral hypothalamus, nucleus r otundus, nucleus sphericus, periventricular nucleus of the hypothalamu s, preoptic area, periventricular nucleus of the preoptic area, septum , striatum, torus semicircularis, and ventromedial nucleus of the hypo thalamus. This is the first demonstration in a vertebrate that factors other than gonadal sex hormones, which arise from the individual's ge netic constitution, can affect the sexual differentiation of the brain . (C) 1997 Wiley-Liss, Inc.