Characterization of oestrogen receptors in zebrafish (Danio rerio)

We cloned the cDNAs corresponding to three oestrogen receptors (ERs) in zebrafish (Danio rerio). Sequence analysis and phylogenetic studies demonstrated that two of these genes, ERbeta.1 and ERbeta.2, arose from duplication of the original ERbeta in many species of the fish phylum, whereas ERalpha is unique. Zebrafish ERs behaved as oestrogen-dependent transcription factors in transactivation assays. However, their reactivity to various oestrogen modulators was different compared with that of mouse ERs. ER mRNA expression during zebrafish development is restricted to distinct time periods, as observed by RNase protection assays. ERbeta.2 is initially expressed as maternally transmitted RNA, until 6 h after fertilization, when expression disappears. Between 6 and 48 h after fertilization, no ER expression could be observed. After 48 h after fertilization, all ERs, but predominantly ERalpha, began to be expressed. We conclude that oestrogen signal transduction can operate during zebrafish development only within discrete time windows.