Nonapeptide molecular evolution during the adaptive radiation of Tanganyika cichlids.

Oxytocin (OT) and vasotocin (VT) are evolutionarily conserved nonapeptides that regulate a wide range of physiological and behavioral processes in vertebrates. Their receptor families have undergone gene duplications that facilitated functional diversification throughout vertebrate evolution. Using the diverse cichlid species in Lake Tanganyika, which have undergone repeated evolutionary transitions between social phenotypes, we investigated the molecular evolution of the nonapeptide system and its potential involvement in social behavior. We performed a positive selection analysis based on the dN/dS ratio and examined the correlation between amino acid variants and two social phenotypes. We also analysed gene expression data to explore associations between brain receptor expression and social phenotype variation. Our findings reveal that, while most sites in nonapeptide receptors are under strong purifying selection, a few sites- primarily in the extended intracellular loop 3 (IL3) of VTR2A receptors- show signatures of positive selection. Additionally, a specific amino acid in VTR2Aa correlates with pair-bonding, suggesting its potential role in social attachment. Gene expression analyses further revealed that components of the nonapeptide system, including VTR2Bb and OT, are differentially expressed across social phenotypes, supporting a role for regulatory variation alongside coding changes. Together, these findings provide new insights into how conserved neuroendocrine systems contribute to social diversity in cichlids.