Steroid receptor phylogeny and vertebrate origins.

Vertebrates appear about 500 million years ago in the fossil record. This is only 25-50 million years after the great explosion of multicellular invertebrate body plans in the early Cambrian. On a geological time scale, this interval is a 'blink of an eye', suggesting that the evolution of regulatory genes is likely to be important in the origins of vertebrates. Here we present evidence for a role of steroid receptors in this process based on a phylogenetic analysis suggesting that receptors for androgens, glucocorticoids, mineralocorticoids, and progesterone evolved from an ancestral steroid receptor gene by two successive duplications over a brief time that could have coincided with the origins of vertebrates. Moreover, the duplications of these steroid receptors may be additional evidence for the two duplications on a genome-scale that have been proposed to be important in the evolution of vertebrates. The two successive duplications of steroid receptor genes and their subsequent sequence divergence leading to steroid-specific receptors that regulate growth, development, reproduction and homeostasis in vertebrates may have been one of the events important in vertebrate survival after the Cambrian during global extinctions that occurred about 440 and 370 million years ago.