A proposed role of the sulfotransferase/sulfatase pathway in modulating yolk steroid effects

Steroid hormones have long been studied by behavioral ecologistsas a nongenetic means whereby females can influence the developmentof their offspring. In oviparous vertebrates, steroids are presentin the yolk at the time of oviposition and have been shown toaffect numerous traits of the offspring. To date, most studieshave focused on the functional relationship between yolk steroidsand offspring development. In this article we used a mechanisticapproach to investigate the effects of yolk steroids in an attemptto decipher how lipophilic steroids may make it from the lipid-richyolk to the developing embryo. First, we examined the distributionof radioactive and nonradioactive estradiol following the exogenousapplication of each to developing eggs of the red-eared slider.Second, we quantified sulfotransferase activity in various componentsof the egg as a potential mechanism for the metabolism of steroids.Results indicate that exogenous estradiol is converted to awater-soluble form during the first 15 days of development,concurrent with an increase of sulfotransferase activity inthe yolk and extra-embryonic membranes. Based on these data,we propose a mechanistic model based upon the sulfotransferase/sulfatasepathway as a means through which developing eggs can convertsteroids to a water-soluble form that can be transported tothe embryo. These sulfonated steroids may then serve as precursorsfor subsequent steroid production via sulfatase activity. Thismodel utilizes a mechanism known to be important for the modulationof maternal steroid signals in placental mammals, at the sametime addressing several previously unanswered questions regardingthe mechanisms underlying the effects of yolk steroids.