Hypothalamic Arginine Vasotocin mRNA Abundance Variation Across Sexes and with Sex Change in a Coral Reef Fish

Abstract

Gonadal hormones are important mediators of sexual and aggressive behavior in vertebrates. Recent evidence suggests that the peptide hormones arginine vasotocin (AVT) and its mammalian homologue arginine vasopressin (AVP) often critically mediate these gonadal hormone effects on behavior and have direct influences on behavioral variation. Behavioral differences between sexes, across reproductive states, and even among closely related species are correlated with differences in central AVT/AVP systems in many species. We report differences in hypothalamic AVT mRNA levels between distinct alternate male phenotypes and with female-to-male sex change in the bluehead wrasse (Thalassoma bifasciatum), a teleost fish. The aggressively dominant and strongly courting male phenotype has greater numbers of AVT mRNA producing cells in the magnocellular preoptic area of the hypothalamus than females. Levels of AVT mRNA within these cells in dominant males are also approximately three times female levels whereas the non-aggressive male phenotype has AVT mRNA levels approximately twice female levels. Behavioral sex change is very rapid in this species and is not dependent on the presence of gonads. Conversely, rapid increases in sexual and aggressive behavior during sex change are closely paralleled by approximate fourfold increases in hypothalamic AVT-mRNA levels. The behavioral plasticity shown by bluehead wrasses in response to social environment might be mediated in part by a neuropeptide, AVT, with changes in the gonads and gonadal hormones as the result rather than the cause of behavioral dominance.

Aromatase immunoreactivity in the bluehead wrasse brain, Thalassoma bifasciatum: Immunolocalization and co-regionalization with arginine vasotocin and tyrosine hydroxylase

“…AVT affects reproductive behaviors in a broad range of vertebrates (Thompson and Moore . Expression of AVT is higher in dominant male bluehead wrasses than females and increases rapidly with sex change (Godwin et al, 2000). This increased expression of AVT is driven by social dominance and is independent of gonads (Semsar and Godwin 2003).…”

Intra-sexual variation in male reproduction in teleost fish: a comparative approach

“…Species with intrasexual variation in reproductive behavior offer the possibility of investigating how AVT may be involved in the differential expression of behavior between male types. In all species studied so far, an association has been found between the expression of alternative tactics and forebrain AVT (i.e., soma size, number of AVT-ir neurons or AVT mRNA expression on a per cell basis, Grober and Bass, 1991;Foran and Bass, 1998;Grober, 1998;Godwin et al, 2000; for a review, see Foran and Bass, 1999).…”

Life History, Neuroendocrinology, and Behavior in Fish

“…Studies in the bluehead wrasse used in situ hybridization to identify cells that were labeled with a degenerate probe that is complementary to AVT mRNA (Godwin et al, 2000). This technique allows an examination of the regulation of AVT production over the course of sex change.…”

“…Rather, TP males appear to have more AVT-mRNAproducing cells and these cells have a higher per cell message production. Godwin et al (2000) sacrificed fish at 2, 3, and 5 days after removal of the TP male from a social group and quantified the level of AVT mRNA expression in each fish. Compared to unmanipulated females, levels of AVT mRNA expression increase progressively 2 and 3 days after the removal of the TP male.…”

“…Decreases in the amount of aggressive behavior received by the largest subordinate result in rapid changes in brain peptide expression (e.g., AVT; Godwin et al, 2000;NPY, Kramer and Imbriano, 1997). These early changes in the brain permit the production of higher rates of male-typical behavior in the early stages of sex change.…”