Population Variation in Steroid Hormone-Related Gene Expression and Sexually Dimorphic Communication in the Electric Fish Apteronotus albifrons.

The development and maintenance of sexually dimorphic behavior often depends on gonadal steroids. Interspecific variation in the magnitude of behavioral sex differences may be mediated by differences in steroid action on neural circuits controlling behavior. The electric communication signals of South American knifefishes are an excellent model to study the evolution and neuroendocrinology of sexually dimorphic behavior because species vary in the sexual dimorphism of these signals and because simple and well-characterized neural circuits regulate the production of electrocommunication signals. Specifically, electric organ discharge frequency (EODf) is controlled by the hindbrain pacemaker nucleus (Pn) and differs between sexes in some species but not in others. Species differences in EODf sexual dimorphism are linked to variation in Pn expression of genes for steroid receptors, steroidogenic enzymes, and receptor cofactors (Proffitt and Smith, 2024). In this study, we asked whether differences in sexual dimorphism of EODf across populations of black ghost knifefish (Apterontus albifrons) are also associated with variation in expression of these genes. Compared to an A. albifrons population with sexually dimorphic EODf, sexually monomorphic A. albifrons populations had reduced Pn expression of genes for androgen receptors and enzymes that increase potency of androgens and had greater expression of genes for a steroid receptor corepressor and for enzymes that reduce gonadal steroid potency. These findings suggest that changes in hormone-related gene expression in brain regions controlling behavior constitute control points for the evolution of variation in sexually dimorphic behavior across populations as well as across species.