| Abstract | Some classes of endocrine disrupting compounds in the environment have the ability to alter thyroid function. Though exposures to thyroid disrupting compounds during early development are known to influence growth and development, recent studies suggest that such exposures can also have adverse effects on reproduction. For example, a recent study demonstrated that early-life stage thyroid disruption caused decreased reproductive output in fathead minnows (Pimephales promelas) even after a prolonged period of depuration. However, the mechanisms connecting early-life stage thyroid disruption to altered reproduction during adulthood remain elusive. This study sought to determine whether alterations in reproductive success following thyroid disruption were a result of altered male or female reproductive performance. Larval fathead minnows less than 1 day post hatch were exposed to the model thyroid inhibitor propylthiouracil (PTU) for 36 days, and after a period of depuration of 80 days, PTU-exposed and control fish were used in a breeding assay which utilized a factorial pair design of male and female fish. Pairs containing PTU-males had reductions in fecundity regardless of female exposure history, indicating thyroid inhibition of male fathead minnows drove the reduction in reproductive output. Since pairs containing male PTU-exposed fish had decreased fecundity, it may be possible that thyroid inhibition during early development alters brain development in a way that leads to improper male sexual behavior. This hypothesis was tested by measuring the expression of genes related to sex steroid signaling in the brains of the fish during development, and the expression in PTU-exposed fish were decreased compared to that of controls. Overall, the results of this study indicate that early-life stage thyroid disruption leads to alterations in male reproductive performance and that such alterations may result from altered patterns of sexual differentiation of the brain and subsequent changes in sexual behavior. |
