Bearded Fireworm
As the primary mediators of estrogen signaling in vertebrates, estrogen receptors (ERs) play crucial roles in reproduction, development, and behavior. They are also the major mediators of endocrine disruption by xenobiotic pollutants that mimic or block estrogen action. ERs that are sensitive to estrogen and endocrine disrupters have long been thought to be restricted to vertebrates: although there is evidence for estrogen signaling in invertebrates, the only ERs studied to date, from mollusks and cephalochordates, have been insensitive to estrogen and therefore incapable of mediating estrogen signaling or disruption. Functional assays show that annelid ERs specifically activate transcription in response to low estrogen concentrations and bind estrogen with high affinity. Furthermore, numerous known endocrine-disrupting chemicals activate or antagonize the annelid ER. This is the first report of a hormone-activated invertebrate ER.