The Effects of Endocrine Disrupting Chemicals On Sex Differentiation in Japanese Medaka (Oryzias latipes) and the Role of Brain Aromatase

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Coastal Sciences, Gulf Coast Research Laboratory

First Advisor

Marius Brouwer

Advisor Department

Coastal Sciences, Gulf Coast Research Laboratory


In fish, the sex determining factors are pliable and can be influenced by environmental factors, including endocrine disrupting chemicals (EDCs), to disrupt sex differentiation. Therefore, the aim of this dissertation was two-fold: (1) establish a simple, cost-effective assay to determine the capability of EDCs, EDC mixtures, and contaminated sediment to elicit reversal of sexual phenotype in medaka ( Oryzias latipes ) and (2) examine a proposed mechanism through which EDCs produce this sexual inversion. To achieve this aim the following hypotheses were tested: (1) the d-rR strain of medaka can be used to examine the ability of EDCs to induce sex inversion upon immersion exposure during critical periods of development, and (2) changes in aromatase activity are needed to invert sex differentiation. Aromatase converts androgens to estrogens contributing to the balance of sex steroids, which are important in endogenous sex differentiation. To examine these hypotheses, medaka were exposed for two weeks after hatch to EDCs including an anti-estrogen, endogenous androgen, environmental estrogen, environmental aromatase inhibitor, pharmaceutical aromatase inhibitor, contaminated sediments, and a mixture of environmentally relevant antagonistic EDCs. Developmental responses demonstrate that exposure to estrogenic compounds lead to complete feminization, but androgen, aromatase inhibitors, and sediments had no effect on sex differentiation. To examine the role of brain aromatase (cyp19b ) in sex inversion, the coding and promoter sequences of the heretofore unknown medaka cyp19b were determined. Analysis of the promoter sequence showed the presence of binding sites for transcription factors involved in primary neurogenesis and estrogenic response. Measurement of cyp19b expression and activity demonstrated that an estrogen mimic can readily induce aromatase gene expression and activity accompanied by male to female sex inversion. However, inhibition of aromatase activity did not prevent an environmental estrogen induced female to male inversion, indicating that increased aromatase activity is not required for male to female sex inversion. It appears that co-exposure to a mixture of environmentally relevant EDCs that are estrogenic and anti-estrogenic in singular treatments, does not prevent the feminizing action of the former. Finally, inhibition of aromatase during ovarian differentiation was inadequate to induce a female to male sex inversion, suggesting that inhibition throughout testis development may be necessary for masculinization.