WHOLE-BODY AND PLASMA-CONCENTRATIONS OF STEROIDS IN THE TURTLE, TRACHEMYS-SCRIPTA, BEFORE, DURING, AND AFTER THE TEMPERATURE-SENSITIVE PERIOD FOR SEX DETERMINATION

To examine endogenous steroid levels during early sexual differentiation in a reptile, we measured whole-body and plasma steroid concentrations in a turtle (Trachemys scripta) throughout embryogenesis using radioimmunoassays. Whole embryos contained testosterone, estradiol, progesterone, and corticosterone before the temperature-sensitive period for sex determination (TSP) (stage 14). The concentration of each of these steroids increased significantly between stage 14 and the initiation of the TSP (stage 16), but no increases were evident between stage 16 and the middle of the TSP (stage 18). No sex differences in steroid content were observed prior to the TSP (stage 14). However, presumptive male embryos (26-degrees-C incubation temperature) contained significantly more testosterone and estradiol than presumptive females (31-degrees-C incubation temperature) during the TSP (stages 16 and 18), and males contained more progesterone than females at stage 18. Plasma samples collected during the TSP (beginning at stage 17+) contained testosterone, estradiol, progesterone, androstenedione, and corticosterone. Progesterone and corticosterone were present in the highest concentrations, followed by androstenedione. Testosterone and estradiol were found in plasma at lower, similar concentrations except at stage 17+, when estradiol levels were approximately 3-fold higher than testosterone levels in both sexes. The only significant sex difference found in plasma steroid levels was for estradiol at stage 19, with males having higher levels than females. Our results demonstrate the presence of a rich steroid milieu throughout early development and support our previous findings that embryonic tissues can synthesize each of these steroids in vitro. Furthermore, a role for sex steroids in the process of sexual differentiation is strongly suggested.