Gonadotropin-Releasing Hormones (GnRHs) in a Primitive Teleost, the Arowana: Phylogenetic Evidence That Three Paralogous Lineages of GnRH Occurred Prior to the Emergence of Teleosts

Abstract

Multiple molecular forms of gonadotropin-releasing hormone (GnRH) are present in a single vertebrate species. To extend the knowledge on GnRH evolution and the number of GnRH forms in one organism, GnRH cDNAs have been isolated and characterized from one of the most primitive teleosts, the arowana Scleropages jardini. This species had two molecular forms of GnRH: salmon-type GnRH (sGnRH) and chicken-II-type GnRH (cGnRH-II). Sequence comparison between the prepro-GnRHs of the arowana and those of other teleosts indicated that sGnRH represented a paralogue separate from any other forms of GnRH. Consistently, subsequent phylogenetic analysis showed that known forms of GnRH in teleosts fell into three paralogous lineages: sGnRH alone on one lineage, cGnRH-II on another, and many other forms on the other. These results suggest that an ancestral GnRH gene duplicated twice prior to the emergence of teleosts and, therefore, that teleosts, and probably also tetrapods, would possess three paralogous forms of GnRH in individual brains.