Effects of incubation temperature and estrogen exposure on aromatase activity in the brain and gonads of embryonic alligators.

Abstract

During embryogenesis, incubation temperature and the hormonal environment influence gonadal differentiation of some reptiles, including all crocodilians. Current evidence suggests that aromatase, the enzyme that converts androgens to estrogens, has a role in sexual differentiation of species that exhibit temperature-dependent sex determination (TSD). During the temperaturesensitive period (TSP) of sex determination, we compared aromatase activity in the brain and gonads of putative male and female alligator embryos to determine if aromatase activity in the embryonic brain could provide the hormonal environment necessary for ovarian development in a TSD species. In addition, we assessed the pattern of aromatase activity in the brain and gonads of embryos treated with estradiol-17β (E 2 ) and incubated at male-producing temperatures to compare enzyme activity in E 2 sex-reversed females to control males and females. This has particular significance regarding wildlife species living in areas contaminated with suspected environmental estrogens. Gonadal aromatase activity remained low during the early stages of the TSP in both sexes and increased late in the TSP only in females. Aromatase activity in the brain increased prior to gonadal differentiation in both sexes. These results suggest that aromatase activity in the brain is not directly responsible for mediating differentiation of the gonad.

Cerebral and gonadal aromatase expressions are differently affected during sex differentiation of Pleurodeles waltl

“…Moreover, embryos incubated at male-producing temperatures and treated simultaneously with estradiol develop as females with an intermediate aromatase activity in gonads and a masculinized brain activity. This does not support an effect of the brain on gonad differentiation in the alligator (Milnes et al 2002). Decapitation experiments in embryos of two other species of reptiles had already suggested that gonadal sex determination is independent of the brain (Raynaud 1962, Dufaure 1966.…”

“…However, the site of temperature action is unknown: does it act directly on the gonad or on another target? In some turtles (Thomas et al 1992, SalameMendez et al 1998, Willingham et al 2000 and crocodilians (Smith & Joss, 1994, Smith et al 1995, Milnes et al 2002, aromatase activity and expression in the gonads do not show any difference between males and females either before the end of TSP, or even later. In such species, differences between male and female embryos have been described in the brain, which is another major aromatase expressing organ.…”

“…The latter determines the steroidogenic environment of the gonads leading to either ovary or testis differentiation, depending on the concentration of estrogens (and androgens). However, in some species, aromatase mRNA is not observed in the genital ridge or in the differentiating gonads during the TSP: the sole aromatase activity detected in the gonad is observed after the temperature-sensitive window in the red-eared slider turtle, Trachemys scripta (Thomas et al 1992, Willingham et al 2000 and Lepidochelys olivacea (Salame-Mendez et al 1998) as well as in the crocodilians Alligator mississipiensis (Smith et al 1995, Milnes et al 2002 and Crocodylus porosus (Smith & Joss 1994). This suggests that during early sex differentiation in these TSD species, steroids could have an extragonadal origin.…”

Temperature-Dependent Sex Determination in Vertebrates

“…The diencephalon, which includes the hypothalamus, had a higher content of testosterone and ability to transform testosterone into 17␤-estradiol at a female-producing temperature than at a male-producing temperature. Aromatase activity in the brain also differed between male-and female-producing temperatures in the slider turtle (Willingham et al 2000), but not in the alligator (Milnes et al 2002). These results reveal additional species differences in temperature effects and underscore the need for caution when proposing mechanisms that may underlie the diverse phenotypic effects produced by incubation temperature.…”

Incubation history prior to the canonical thermosensitive period determines sex in the American alligator

“…The authors attributed the reported discrepancies between their findings and those reported previously to thermal features of the shift regimes including the magnitude of the temperature shift and the use of a MPT (34 8C) that they recognized to produce both males and females (Lang & Andrews 1994). Since the publication of this report, the period of development between embryonic stages 21 and 24 has served as the canonical period of thermosensitivity in alligator, and numerous studies have incorporated this specific period into designs aimed at characterizing the origins and mechanisms of TSD (Conley et al 1997, Western et al 1999a, Gabriel et al 2001, Milnes et al 2002.…”