Functional distinctions associated with the diversity of sex steroid hormone receptors ESR and AR

Sex steroid hormones including estrogens and androgens play fundamental roles in regulating reproductive activities and they act through estrogen and androgen receptors (ESR and AR). These steroid receptors ha, evolved from a common ancestor in association with several gene duplications. In most vertebrates, this la resulted in two ESR subtypes (ESR1 and ESR2) and one AR, whereas in teleost fish there are at least three ESR (ESR1, ESR2a and ESR2b) and two ARs (AR alpha and AR beta) due to a lineage-specific whole genome duplication Functional distinctions have been suggested among these receptors, but to date their roles have only be characterized in a limited number of species. Sexual differentiation and the development of reproductive organ are indispensable for all animal species and in vertebrates these events depend on the action of sex steroid hormones. Here we review the recent progress in understanding of the functions of the ESRs and ARs in the development and expression of sexually dimorphic characteristics associated with steroid hormone signaling vertebrates, with representative fish, amphibians, reptiles, birds and mammals.