Nuclear receptor superfamily members are metazoan innovations which are present in all living metazoan phyla. These genes, presumably originating from a common ancestor in an extinct phylum that predates the sponge (Porifera), has evolved, and is still evolving, to mediate nearly every facet of metazoan life. Representatives of all six classical nuclear receptor superfamily members were present before the protostomedeuterostome split, more than 700 million years ago. Nuclear receptor sequence and structure diversification to accommodate ligand binding has occurred numerous times, with the earliest report of steroid-hormone binding occurring in the Annelid and of retinoic acid in the lower metazoan phylum Cnidaria. During the Cambrian period a quadruplication of the genome leading to jawed vertebrates gave rise to many of the nuclear receptor paralogues present today.
Key Concepts:
- The evolutionary origins and diversification of the nuclear receptor superfamily gene structure in the Metazoa.
- Nuclear receptors are unique to the metazoan kingdom.
- Members of the nuclear receptor superfamily are present in all extant phyla of metazoans.
- Representatives of six classical nuclear receptor superfamily members were present before the proteosomedeutrosome division more than 700 million years ago.
- During the Cambrian period a quadruplication of the genome giving rise to jawed vertebrates occurred producing many of the nuclear receptor paralogues present today.
Keywords: nuclear receptor; steroid hormone; Cambrian period; molecular exploitation; Metazoa; placozoa








