Sex reversal and aromatase in the European pond turtle: treatment with letrozole after the thermosensitive period for sex determination.

In the European pond turtle (Emys orbicularis), gonadal sex differentiation is temperature-dependent. The temperature sensitive period (TSP) of gonadogenesis lies between stages 16 and 22 of embryonic development. Previous studies have shown that embryos incubated at 30 degrees C, a temperature yielding 100% phenotypic females, can be sex reversed by treatments with an aromatase inhibitor administered during TSP or even somewhat after TSP (as of stage 22+). The goal of the present study was to determine whether the ovary still retains male potential at later stages of embryonic development and whether the induced male characters persist after hatching. For this purpose, eggs of E. orbicularis were treated with letrozole, a nonsteroidal aromatase inhibitor, at or as of stages 23, 24 or 25, then gonadal aromatase activity in each individual and the related gonadal structure were studied at hatching (stage 26) and for one year after hatching. Two kinds of treatments were carried out: 1) repeated applications of 10 microg of letrozole in ethanolic solution onto the eggshell; and 2) a single injection of 10 microg of letrozole in olive oil. Similar results were obtained with either application or injection of the aromatase inhibitor. In treatments as of or at stage 23, individuals with gonadal aromatase activity lower than 20 fmoles/hour/gonad had ovotestes, i.e., 22% of the treated individuals. At hatching, the inner part of these ovotestes contained testicular cords and also mixed lacunae presenting various degrees of transdifferentiation of the epithelium into a Sertolian epithelium. The cortex was maintained, although some germ cells degenerated within it. These processes continued after hatching. However, at 12 months, gonads were still ovotestes displaying some follicles with a growing oocyte in the remaining parts of the cortex. In treatments as of or at stages 24 or 25, only a few individuals were masculinized. One had ovotestes; in others, the cortex was absent in some parts and when it was present oocytes were degenerating. These results show that in the European pond turtle, differentiation of ovotestes from ovaries can be induced by treatment with an aromatase inhibitor starting at late stages of embryonic development (between the end of TSP and hatching), although such differentiation is less frequent as embryonic development proceeds. Sex reversal persists for at least one year after hatching. J. Exp. Zool. 290:490-497, 2001.