Molecular characterization of estrogen receptors 1, 2a, and 2b and their tissue and ontogenic expression profiles in fathead minnow (Pimephales promelas). Biol Reprod 73: 648–662

There are two estrogen receptor (ER) subtypes in fish, Esr1 and Esr2 (formerly ERa and ERb), and in some species the Esr2 subtype has two forms, Esr2b (formerly ERb1) and Esr2a (for-merly ERb2 or ERg). There is little information, however, on the different characteristics and functional significance of the two receptor subtypes in fish, and this is especially relevant for un-derstanding the disruption of ER signaling by chemicals with estrogenic activity. In this study, the full-length cDNAs for esr1 (3167 base pairs [bp]) and esr2b (2318 bp), and a partial-length (267 bp) cDNA for esr2a, were cloned and characterized in fat-head minnow (fhm; Pimephales promelas), and their patterns of expression established during development and in adults. Real-time polymerase chain reaction revealed some clear distinctions in the ontogenic and tissue expression of fhm esr1, esr2b, and esr2a, suggesting different functions for each ER subtype. Fhm ERs were expressed in brain, pituitary, liver, gonad, intestine, and gill of male and female fish, esr2b and esr2a were also ex-pressed in muscle. Fhm esr1 and esr2b were expressed predom-inantly in the liver, whereas fhm esr2a was expressed predomi-nantly in intestine and was lowest expressed in liver. Responses of the different hepatic ERs in male fathead minnow exposed to 100 ng estradiol/L differed, with a significant induction (5-fold) of fhm esr1 but no effect on esr2b or esr2a expression, sug-gesting different mechanisms of regulation for the different ERs. The detailed characterization of ERs in fathead minnow provides the foundation for understanding the molecular basis of estro-genic disruption in fish. developmental expression, endocrine disruption, estrogen recep-tors, fathead minnow, real-time polymerase chain reaction, tissue expressio