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% pheno typic females are obtained. Many data argue in favor of the involvemen t of steroid hormones in this phenomenon. During the thermosensitive p eriod for sex determination, gonads are able to metabolize different s teroid precursors, such as pregnenolone, progesterone, androstenedione , and dehydroepiandrosterone. Gonadal oestrogen content is higher at 3 0 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 degr ees C before or at the beginning of the thermosensitive period, gonads differentiate into ovaries instead of testes: an ovarian-cortex devel ops whereas medullary testicular cords are inhibited. Conversely, when tamoxifen (an anti oestrogen) is injected into eggs incubated at 30 d egrees 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 degree s C. When eggs are first incubated at 25 degrees C up to a stage withi n this period, and then shifted to 35 degrees C (a highly feminizing t emperature) for different times, the response is also exponential, sho wing an amplification of aromatase synthesis. Temperature could act, d irectly or indirectly, on the regulation of the expression of the arom atase gene. The anti-Mullerian hormone (AMH) exerts an inhibitory effe ct on aromatase activity. The termination of the thermosensitive perio d could result in testes from an active synthesis of AMH, and in ovari es from high levels of oestrogens. (C) 1994 Wiley-Liss, Inc.