Developmental synergism of steroidal estrogens in sex determination.

Abstract

Gonadal sex in the red-wared slider turtle, Trachemys scdpta, 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. Specificlly, nonaromatizable androgens and aromatase inhibitors induce testis dif tiain 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 feiales than would be expected ifestrogens and temperaure had purely addiie eflects 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, 17JB-estradiol, and estriol. A low dose of 171-estradiol (200 ng) showed significant synergism when administered with a single low dose of estriol (10 ng [Online 8 January 1999] hap:// ehpnett.nies.nib.gov/docs/1999/107p93-97bffgeron/abstract.html A number of man-made compounds mimic estrogens, although with a lower potency than natural steroidal estrogens (1-3). When considered individually, these chemicals may exist in the environment in concentrations too low to be of concern. In combination, however, low dosages of these same compounds may act synergistically to produce a strong estrogenic response. This low-dose synergy was first shown with polychlorinated biphenyls (PCBs) using an in vivo sex-reversal assay in the red-eared slider turtle (Trachemys scripta) (4). Although the pesticides endosulfan and dieldrin were reported to produce similar synergy using a yeast gene expression system (5), subsequent studies failed to replicate this result (6,7). Thus, controversy still surrounds environmental estrogens and whether they exhibit synergistic activity. This is an especially important question because low dosages of PCBs and pesticides could pose a health risk through inappropriate activation of estrogen-regulated processes. Indeed, naturally occurring steroidal estrogens play a prominent role in early development and in adult reproductive function through their effects on cell differentiation and cellular organization of the ovary (8-10).The developmental effects of steroidal estrogens are even more pronounced in the red-eared slider turtle because sex steroids have been implicated in the process of temperature-dependent sex determination (11). Gonadal sex in this and many other reptiles is determined by embryonic incubation temperature. In this particular species, eggs incubated at constant temperatures below 28.60C develop as males, eggs incubated at or above 29.60C develop as females, and increasing temperature within the narrow 28.6-29.60C range produces increasing proportions of females (12). Moreover, temperature has its effect during a critical period of deve...

Topical dose delivery in the reptilian egg treatment model

“…Sixteen of the 45 studies employing topical exposure or egg injection did not perform statistical analysis on control versus treatment sex ratios (Table S1, Supplemental Data [http://dx.doi.org/10.1897/06-290.S1]) [8,11,15,16,18,19,21,22,25,26,28,29,34,37,60,61]. Eleven of the 45 studies presented exposure-response data mislabeled or misinterpreted as doseresponse data [8,17,18,23,24,26,27,29,30,32,62].…”

“…Sixteen of the 45 studies employing topical exposure or egg injection did not perform statistical analysis on control versus treatment sex ratios (Table S1, Supplemental Data [http://dx.doi.org/10.1897/06-290.S1]) [8,11,15,16,18,19,21,22,25,26,28,29,34,37,60,61]. Eleven of the 45 studies presented exposure-response data mislabeled or misinterpreted as doseresponse data [8,17,18,23,24,26,27,29,30,32,62]. Of those that discussed dose-response, five provided statistical analysis on each treatment versus control [23,24,27,32,62], six calculated regression curves [18,23,26,27,32,61], and three did both [23,27,32].…”

“…Eleven of the 45 studies presented exposure-response data mislabeled or misinterpreted as doseresponse data [8,17,18,23,24,26,27,29,30,32,62]. Of those that discussed dose-response, five provided statistical analysis on each treatment versus control [23,24,27,32,62], six calculated regression curves [18,23,26,27,32,61], and three did both [23,27,32].…”

“…Reported responses to in ovo exposures can be summarized based on the class of the applied compound. In general, the percentage of female hatchlings have been reported to increase following exposure during the TSP to various endogenous androgens [13,15,17,22,23,31,33], endogenous and synthetic estrogens [8,11,[13][14][15][16][17][18][20][21][22][23][24][25][26][27][28][29][30][31]37,52,53,58,60,63,64], and endogenous and synthetic progestogens [22,23] at temperatures typically resulting in male-only, male-biased, 1:1, and femalebiased sex ratios. Nonaromatizable androgens have been reported to increase the percentage of males when applied to eggs incubated at female-biased and 1:1 sex ratio-producing temperatures [19,23,31,32].…”

“…Thus, the presence of exogenous hormone or hormone agonist/antagonist in the egg could alter normal endocrine function and, ultimately, influence gender determination. Indeed, exposure-dependent increases in the proportion of females were observed when reptilian eggs were treated with 17␤-estradiol during incubation at male-producing temperatures [8,11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. At incubation temperatures that typically produce a 1:1 sex ratio, treatment with nonaromatizable androgens increased the proportion of males [19,23,31,32].…”