AROMATASE ENZYME-ACTIVITY DURING GONADAL SEX-DIFFERENTIATION IN ALLIGATOR EMBRYOS

Sexual differentiation of the gonads in Alligator mississippiensis and many other oviparous reptiles is controlled by egg incubation temperature. Estrogens are thought to play a role in this process, and it has been hypothesized that estrogen production is thermosensitive in species with temperature-dependent sex determination (TSD). Using the tritiated water assay, we measured the activity of the critical estrogen-synthesising enzyme, P450 aromatase, in the gonad-adrenal-mesonephric kidney complex (GAM) of alligator embryos incubated at male- and female-producing temperatures. Aromatase activity increased in the GAM of developing embryos incubated at 30 degrees C (100% female-producing) and 34.5 degrees C (predominantly female-producing), while it remained very low throughout development in embryos incubated at the intermediate temperature of 33 degrees C (100% male-producing). However, it is unclear whether enhanced aromatase activity represents the initial signal for ovary differentiation or whether it lies downstream in the female developmental pathway. For embryos incubated at 30 degrees C (female-producing), there was no detectable increase in aromatase activity until developmental stage 24, which is after the temperature-sensitive period for sex determination. This suggests that aromatase may be a downstream component of the ovary-determining cascade. In female alligator hatchlings, most of the aromatase activity was localised in the ovary, activity being low in the adrenal-mesonephros. Aromatase assays carried out at 30 degrees C and at 33 degrees C indicated that, at viable incubation temperatures, aromatase activity is not thermosensitive. This suggests that temperature directly or indirectly influences enzyme synthesis. Radioimmunoassay of estradiol synthesised by the GAM during the aromatase assay confirmed increased enzyme activity during female development but not during male development. Increased aromatase activity and estrogen synthesis during female development were correlated with the timing of ovary differentiation, particularly proliferation of the gonadal cortex. These findings implicate aromatase in temperature-dependent gonadal sex differentiation in alligator embryos, higher enzyme activity being associated with ovary development.