Vasoactive intestinal peptide as a mediator of the effects of a supergene on social behavior

Supergenes, or linked groups of alleles that are inherited together, present excellent opportunities to understand gene-behavior relationships. In white-throated sparrows (Zonotrichia albicollis), a supergene on the second chromosome associates with a more aggressive and less parental phenotype. This supergene includes the gene for vasoactive intestinal peptide (VIP), a neuropeptide known to play a causal role in both aggression and parental behavior. Here, using a free-living population, we compared levels of VIP mRNA between birds with and without the supergene. We focused on the anterior hypothalamus and infundibular region, two brain regions containing VIP neurons known to play a causal role in aggression and parental behavior, respectively. First, we show that the supergene enhances VIP expression in the anterior hypothalamus and that expression positively predicts vocal aggression independently of genotype in both sexes. Next, we show that the supergene reduces VIP expression in the infundibular region, which suggests reduced secretion of prolactin, a pro-parental hormone. Thus, patterns of VIP expression in these two regions are consistent with the enhanced aggression and reduced parental behavior of birds with the supergene allele. Our results illustrate mechanisms by which elements of genomic architecture, such as supergenes, can contribute to the evolution of alternative behavioral phenotypes.