Sulfonation of maternal steroids is a conserved metabolic pathway in vertebrates

All vertebrate embryos develop in the presence of maternally derived steroids, and maternal steroids have been hypothesized to link phenotype of the offspring to maternal physiology. In placental vertebrates, it is known that maternally derived steroids are metabolized during development via the sulfonation pathway. We used eggs from the red-eared slider turtle (Trachemys scripta) to determine whether the same metabolic pathway is used to metabolize maternally derived steroids in an oviparous vertebrate. To examine the relationship between estradiol and estrogen sulfates during development, levels of maternally derived estradiol were compared with levels of estradiol sulfate, estrone sulfate, and estriol sulfate at oviposition and after 20 days of embryonic development. Estrone sulfate was the only detectable estrogen sulfate. At oviposition, levels of both estradiol and estrone sulfate varied seasonally with clutches from later in the nesting season having significantly higher concentrations of both steroids. Levels of estrone sulfate increased during development, demonstrating that the sulfonation of maternally derived steroids occurs in oviparous vertebrates as well as in placental vertebrates. We also found that exogenous estrone sulfate increases the production of female hatchlings, thereby demonstrating the ability of this metabolite to influence embryonic development. To examine the role of sulfonation in the metabolism of maternal progesterone and testosterone, we characterized the metabolic fate of both steroids by applying tritiated forms of each steroid at oviposition and characterizing metabolites after 20 days of incubation. Similar to what was demonstrated for estradiol, both progesterone and testosterone are converted to sulfonated metabolites during embryonic development. These data suggest that steroid sulfates, both those that are maternally derived and those resulting from the metabolism of maternal steroids, are a key component of the mechanism underlying steroid-mediated maternal effects.