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¹ Department of Biological Sciences, Florida Atlantic University, 5600 US 1, North, Ft. Pierce, Florida 34946, USA
² Department of Bioenvironmental Research, Okazaki Institute for Integrative Bioscience, National Institutes of Natural Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan

(Requests for offprints should be addressed to E F Orlando; Email: eorlando{at}fau.edu)

The mechanisms underlying sex determination and differentiation in fishes are labile in response to environmental parameters. Sex-specific phenotypes are largely regulated by sex steroids, and the inhibition or the stimulation of aromatase can reverse sex as well as alter secondary sexual characteristics in fishes. Among vertebrates, the mangrove rivulus is the only known self-fertilizing hermaphrodite. Throughout most of its range, rivulus appear to exist as clonally reproducing hermaphrodites. However, outcrossing has been documented in Belize, where up to 25% of rivulus collected are males. The direct development of (primary) males occurs when embryos are incubated at 18 °C and hermaphrodites develop into secondary males when held at 28 °C. Given the importance of sex steroids, their receptors, and aromatase in sex determination and differentiation of fishes, we cloned, sequenced, and quantified the expression of estrogen receptors (ER, ERß) and ovarian (AroA) and brain (AroB) aromatase genes. Hermaphrodites had increased ER, ERß, AroA, and AroB gene expression in the liver, gonad, gonad, and brain respectively, compared to males. These data are consistent with the gene expression data reported for other species and are reflective of the presence of ovarian tissue in the hermaphrodites. Interestingly, we show the elevated expression of brain aromatase in the hermaphrodite brain. The role of the dimorphic expression of brain aromatase in the regulation of sex-specific characteristics is intriguing and requires further research. Because of the uniqueness of its reproductive biology, rivulus is an excellent model for elucidating the mechanisms regulating vertebrate sex determination and sexual differentiation.