Time required for temperature-induced changes in gonadal aromatase activity and related gonadal structure in turtle embryos

Abstract. In the turtle Emys orbicularis , sexual differentiation of gonads is temperature-dependent. Oestrogens have been shown to be involved in this phenomenon and temperature has been expected to act, directly or indirectly, on regulation of synthesis or activity of cytochrome P-450 aromatase (P-450 arom). We have studied the effects of temperature shifts and of exposure at female- or male-producing temperatures for different times on gonadal aromatase activity and gonadal structure. In a first series of experiments, eggs were incubated at 25°C (masculinizing temperature) up to stage 18 and then exposed for 1 to 8 days at 35°C, a highly feminizing temperature. The response was exponential: aromatase activity increased clearly only after 4 day exposure at 35°C, then it was considerably enhanced. After 1 and 2 days at 35°C, the structure of gonads was not modified. With longer exposures at 35°C, gonads were progressively feminized: medullary epithelial cords disappeared, whereas an ovarian cortex was forming. In another type of experiment, eggs incubated at 30°C (feminizing temperature) until stage 19 were transferred at 25°C for 6 days. In embryos of these shifted eggs, gonadal aromatase activity was about ninefold lower than that in control embryos (maintained at 30°C). However, this activity did not fall to the level measured in embryos of the same stage incubated at 25°C from egg-laying and was about twofold higher than that measured at the time of transfer. Gonads exhibited a cortex anlage but the medulla was more voluminous than that of controls and epithelial cords were beginning to form within. Together these results show that changes in gonadal aromatase activity and in gonadal structure are correlated, and that temperature acts on regulation of P-450 arom synthesis. Amplification of this synthesis during the thermosensitive period at higher temperatures could reflect amplification of expression of the P-450 arom gene.