Preoptic GnRH and AVT: axes for sexual plasticity in teleost fish

Alternative reproductive tactics within one sex, adult sex or role change, and reproductive suppression are all forms of reproductive plasticity commonly exhibited among teleost fishes. The two neuropeptides that have been most extensively studied with regard to such behavioral plasticity are gonadotropin releasing hormone (GnRH) and arginine vasotocin (AVT). Here, we review intra- and intersexual variation in the number and size of GnRH and AVT neurons along with gonadal phenotype in those species of teleosts showing intraspecific plasticity in reproductive behavior. In several species, male dimorphisms in the number and/or size of GnRH neurons in the forebrain's preoptic area parallel a divergence in relative gonad size and reproductive tactics. The available studies of AVT-containing neurons in the preoptic area also indicate intrasexual dimorphisms among males, although a proximate link to other reproductive traits and behavioral outcomes is more difficult to recognize. For both GnRH and AVT, there are also species-typical patterns in the coupling between structural (e.g., neuronal and gonadal) traits and reproductive tactic expressed, which likely reflect distinct patterns of adaptation to particular ecological environments. As discussed, neurophysiological, biochemical, and receptor density studies are now essential to establish the functional significance of the diverse organizational patterns of GnRH and AVT neurons in teleosts. Similar studies also need to be carried out in species of other vertebrate groups that show comparable behavioral plasticity.