Sensitive Stages for the Effects of Temperature on Gonadal Aromatase Activity in Embryos of the Marine Turtle Dermochelys coriacea

Abstract

Many data suggest an involvement of estrogens in gonadal differentiation in reptiles with temperature-dependent sex determination (TSD). However, the site of estrogen synthesis in two species of freshwater turtles is unclear. In Emys orbicularis , estrogens were shown to be produced by the gonads, whereas in Trachemys scripta, gonadal steroids were not detected. The marine turtle Dermochelys coriacea exhibits TSD but in gonadal development, ovarian differentiation is delayed. Gonadal aromatase activity and estrogen content in this species were measured in embryos incubated at 27° and in embryos incubated at 30.5°, respectively, masculinizing and feminizing temperatures within the range of temperatures found in natural nests. At all stages studied, aromatase activity was present and found to be higher at 30.5° than at 27°. Estrogens were only found at 30.5°. The effects of temperature shifts on gonadal aromatase activity were then examined. Eggs were shifted from 27 to 35° (feminizing temperature) at different embryonic stages and exposed to 35° for 6 days. An increase in gonadal aromatase activity, although with significant individual variations, was seen only when eggs were shifted between stages 23 and 27. These stages are in the range of the thermosensitive stages for sexual differentiation of the gonads determined in other turtles. These results are similar to those previously obtained in E. orbicularis and agree with a key role for endogenous estrogens in gonadal differentiation of reptiles with TSD. Relatively high levels of these hormones are probably required to inhibit the development of testicular cords in all species, but the sensitivity to estrogens of the surface epithelium of the gonads varies with species.