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Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
¹ Department of Biology, Boston University, Boston Massachusetts 02215, USA

(Requests for offprints should be addressed to A M Tarrant; Email: atarrant{at}whoi.edu)

The estrogen receptor-related receptors (ERRs) are a group of nuclear receptors that were originally identified on the basis of sequence similarity to the estrogen receptors. The three mammalian ERR genes have been implicated in diverse physiological processes ranging from placental development to maintenance of bone density, but the diversity, function, and regulation of ERRs in non-mammalian species are not well understood. In this study, we report the cloning of four ERR cDNAs from the Atlantic killifish, Fundulus heteroclitus, along with adult tissue expression and estrogen responsiveness. Phylogenetic analysis indicates that F. heteroclitus (Fh)ERR is an ortholog of the single ERR identified in mammals, pufferfish, and zebrafish. FhERRßa and FhERRßb are co-orthologs of the mammalian ERRß. Phylogenetic placement of the fourth killifish ERR gene, tentatively identified as FhERRb, is less clear. The four ERRs showed distinct, partially overlapping mRNA expression patterns in adult tissues. FhERR was broadly expressed. FhERRßa was expressed at apparently low levels in eye, brain, and ovary. FhERRßb was expressed more broadly in liver, gonad, eye, brain, and kidney. FhERRb was expressed in multiple tissues including gill, heart, kidney, and eye. Distinct expression patterns of FhERRßa and FhERRßb are consistent with subfunctionalization of the ERRß paralogs. Induction of ERR mRNA by exogenous estrogen exposure has been reported in some mammalian tissues. In adult male killifish, ERR expression did not significantly change following estradiol injection, but showed a trend toward a slight induction (three- to five-fold) of ERR expression in heart. In a second, more targeted experiment, expression of ERR in adult female killifish was downregulated 2.5-fold in the heart following estradiol injection. In summary, our results indicate that killifish contain additional ERR genes relative to mammals, including ERRß paralogs. In addition, regulation of ERR expression in killifish apparently differs from regulation in mammals. Together, these features may facilitate determination of both conserved and specialized ERR gene functions.