Peptide hormones versus steroid hormones: case studies from snail and turtle populations

The proposed mechanisms of masculinization in gastropod mollusks (imposex) and feminization in oviparous vertebrates has focused primarily on the interaction of xenobiotics with the steroid hormone system. The evidence does not support the proposed mechanism of imposex induction, inhibition of CYP 19 (aromatase), since the changes in steroid titers occur only in the later stages of imposex. In addition, the role of vertebrate sex steroid hormones is limited in gastropods. Recent evidence suggests that peptide hormones play a key role in masculinization in snails and may be a more plausible mechanism of imposex induction. Steroid hormones play a role in feminization of oviparous vertebrates. Vitellogenin (Vtg, egg yolk) induction has been used extensively as a biomarker of feminization. However, in some species of turtles there is evidence that high levels of estradiol are necessary to initially induce Vtg. Once this imprinting occurs, lower levels of estradiol can then induce high levels of Vtg. It was found that environmental levels of estradiol do not always induce Vtg in male and juvenile turtles. However, these same levels of estradiol significantly elevate female Vtg levels. Elevated female Vtg may lead to larger, but fewer, eggs, and therefore fewer offspring. However, these offspring may have better survivorship. Alternatively, elevated Vtg may lead to a larger number of smaller eggs, which have been shown to produce smaller hatchlings with reduced survival. The effects of elevate female Vtg on fitness are difficult to assess in such long-lived species.