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Characterization of early gonadal development in smallmouth bass (Micropterus dolomieu) and effects of ethinyl estradiol on gonadal differentiation
Citation:
Kadlec, S., R. Johnson, Dave Mount, AND P. Schoff. Characterization of early gonadal development in smallmouth bass (Micropterus dolomieu) and effects of ethinyl estradiol on gonadal differentiation. SETAC North America, Salt Lake City, UT, November 01 - 05, 2015.
Impact/Purpose:
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Description:
Teleosts vary widely in patterns of gonadal sex differentiation, which might lead to differences in how gonadal development is affected by the presence of estrogenic compounds. This makes it difficult to apply our knowledge of model species such as medaka and fathead minnow to other species. As an example, field surveys have observed testicular oocytes in wild-caught smallmouth bass, but it is not clear to what degree these findings imply environmental exposure to estrogenic compounds. As a first step toward better understanding these connections, we studied the development of gonadal tissue in smallmouth bass fry that were reared in clean water, or in a low concentration of a model estrogen (2 ng/L 17-á-ethinyl estradiol, EE2) for 90 d beginning at swim-up, followed by 90 d in clean water. This poster presents a pictorial time series of our histological evaluation of gonadal development in the presence and absence of estrogenic exposure. In controls, the onset of gonadal sex differentiation occurred earlier in putative ovaries than in testes. Subtle indications of ovarian differentiation were observed at day 32, with a stronger distinction from undifferentiated (presumably male) gonads by day 49, and unambiguous ovarian organization by day 63. Testicular differentiation was not evident until day 92. In EE2-exposed groups, 100% of fish developed according to the ovarian pattern by day 63, with continued development of ovaries even after cessation of EE2 exposure, although at days 120 through 180, roughly half of the EE2-exposed fish showed slightly abnormal ovarian structure. This phenotype was characterized by a smaller cross-sectional area, fewer oocytes, and more fibrotic tissue than control ovaries. We speculate that this sub-category of the ovarian phenotype may have been exhibited by genetically male fish that were feminized by EE2 exposure, although genetic sex was not known. Because no EE2-exposed fish followed a path of testicular development, the likelihood that estrogenic exposure leads to testicular oocytes could not be evaluated.