TEMPERATURE SENSITIVITY OF SEXUAL-DIFFERENTIATION OF GONADS IN THE EUROPEAN POND TURTLE - HORMONAL INVOLVEMENT

In the European pond turtle, the incubation of eggs below 28 degrees C yields 100% phenotypic males, whereas above 29.5 degrees C 100% phenotypic females are obtained. Many data argue in favor of the involvement of steroid hormones in this phenomenon. During the thermosensitive period for sex determination, gonads are able to metabolize different steroid precursors, such as pregnenolone, progesterone, androstenedione, and dehydroepiandrosterone. Gonadal oestrogen content is higher at 30 degrees C than at 25 degrees C. When oestradiol benzoate (5 to 50 mu g) or oestrone (100 mu g) are injected into eggs incubated at 25 degrees C before or at the beginning of the thermosensitive period, gonads differentiate into ovaries instead of testes: an ovarian-cortex develops whereas medullary testicular cords are inhibited. Conversely, when tamoxifen (an anti oestrogen) is injected into eggs incubated at 30 degrees C, medullary testicular cords (or tubes) differentiate. Gonadal aromatase activity during the thermosensitive period increases in an exponential fashion at 30 degrees C, and remains very low at 25 degrees C. When eggs are first incubated at 25 degrees C up to a stage within this period, and then shifted to 35 degrees C (a highly feminizing temperature) for different times, the response is also exponential, showing an amplification of aromatase synthesis. Temperature could act, directly or indirectly, on the regulation of the expression of the aromatase gene. The anti-Mullerian hormone (AMH) exerts an inhibitory effect on aromatase activity. The termination of the thermosensitive period could result in testes from an active synthesis of AMH, and in ovaries from high levels of oestrogens. (C) 1994 Wiley-Liss, Inc.