A model for social control of sex change: interactions of behavior, neuropeptides, glucocorticoids, and sex steroids.

The optimal regulation of vertebrate sexual development and reproductive function involves integration of internal physiological signals, indicative of an individual's sexual status and capability for reproduction, with signals from the external environment. While these environmental cues are diverse, and oftentimes species-specific, the induction of sexual readiness is typically carried out through the same basic components of the hypothalamic-pituitary-gonadal axis conserved among vertebrates. Therefore, species exhibiting diverse patterns of reproduction can contribute to the understanding of the general mechanisms underlying the expression of adult sexual phenotypes. The bluehead wrasse, Thalassoma bifasciatum, is a tropical coral reef fish that displays social control of sex change, whereby dominant males inhibit sex change in other members of the social group using aggressive interactions. In many fish species and vertebrates in general, individuals that lose these interactions often experience increased serum glucocorticoids, which can have a subsequent impact on their physiology and behavior. We discuss glucocorticoid regulation of both neuropeptide gene transcription and the major steroid biosynthetic pathways as potential mechanisms involved in the regulation of sex change in the bluehead wrasse. We present a model describing behavioral regulation of sex change in the bluehead wrasse and then describe the potential mechanistic roles of glucocorticoids, gonadal steroids, and neuropeptides in generating the changes predicted by the model. Through the use of alternative model systems it is possible to observe novel interactions among the neuroendocrine axes that regulate major life history events, like reproduction. These insights may then shed light on similar functional mechanisms underlying behavioral regulation of reproduction in all vertebrates.