| Developmental Synergism of Steroidal Estrogens in Sex Determination Judith M. Bergeron, Emily Willingham, C. Todd Osborn, III, Turk Rhen, and David Crews Institute of Reproductive Biology, Department of Zoology, University of Texas at Austin, Austin, TX 78712 USA Abstract
Gonadal sex in the red-eared slider turtle, Trachemys scripta, is determined by incubation temperature during embryonic development. Evidence suggests that temperature determines sex by influencing steroid hormone metabolism and/or sensitivity: steroidogenic enzyme inhibitors or exogenous sex steroid hormones and their man-made analogs override (or enhance) temperature effects on sex determination. Specifically, nonaromatizable androgens and aromatase inhibitors induce testis differentiation at female-producing temperatures, whereas aromatizable androgens and estrogens induce ovary differentiation at male-producing temperatures. Moreover, natural estrogens and temperature synergize to produce more females than would be expected if estrogens and temperature had purely additive effects on sex determination. In this study, we use sex reversal of turtle embryos incubated at a male-producing temperature to examine synergism among steroidal estrogens: estrone, 17ß-estradiol, and estriol. A low dose of 17ß-estradiol (200 ng) showed significant synergism when administered with a single low dose of estriol (10 ng) . Likewise, a single low dose of estrone (250 ng) had a synergistic effect when combined with the same low dose of estriol (10 ng) . We conclude that the weak natural estrogens estrone and 17ß-estradiol synergize with a low dose of the more potent estriol to reverse gonadal sex during the critical period of sexual differentiation. These results suggest that weak environmental estrogens may also synergize with stronger natural estrogens. Key words: estradiol, estriol, estrogen, estrone, synergy, temperature-dependent sex determination. Environ Health Perspect 107:93-97 (1999) . [Online 8 January 1999] http://ehpnet1.niehs.nih.gov/docs/1999/107p93-97bergeron/ abstract.html Address correspondence to D. Crews, Institute of Reproductive Biology, Department of Zoology, University of Texas at Austin, Austin, TX 78712 USA. The authors wish to thank R. Vohra and Y. Tasnif for technical assistance, J. Branch for graphics, and J.A. McLachlan and S.F. Arnold for discussions on the research design and interpretation of the results. This work was partially supported by NIH T32 MH18837, NIMH Research Scientist Award, and NSF IBN 9723617 to D.C. Received 6 May 1997 ; accepted 20 August 1998.
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