| Resumo: |
Although, as their names imply, estrogen receptors [ERs] and estrogen-related receptors [ERRs] are related transcription factors, their evolutionary relationships to each other are not fully understood. To elucidate the origins and evolution of ERs and ERRs, we searched for their orthologs in the recently sequenced genome of _Trichoplax_, the simplest known animal, and in the genomes of three lophotrochozoans: _Capitella_, an annelid worm, _Helobdella robusta_, a leech, and _Lottia gigantea_, a snail. BLAST searches found an ERR in _Trichoplax_, but no ER. BLAST searches also found ERRs in all three lophotrochozoans and invertebrate-like ERs in _Capitella_ and _Lottia_, but not in _Helobdella_. Unexpectedly we find that the _Capitella_ ER sequence is closest to ER[beta], unlike the other invertebrate ER sequences, which are closest to ER[alpha]. Our database searches and phylogenetic analysis indicate that invertebrate ERs evolved in a lophotrochozoan and steroid-binding ERs evolved in a deuterostome.
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