Vasotocin and Oxytocin Receptor Evolution Linked to Social Diversity in Tanganyika Cichlids

Oxytocin (OT) and vasotocin (VT) are conserved nonapeptides crucial for regulating diverse physiological and behavioral processes, including social behaviors in vertebrates. Their receptor families have undergone gene duplications, enabling functional diversification. In the context of social behavior and adaptive radiation, understanding how these neuroendocrine systems evolve can shed light on the mechanisms driving behavioral diversity. The diverse cichlid species in Lake Tanganyika, known for repeated evolutionary transitions in social phenotypes, offer an excellent model to explore the molecular evolution of the nonapeptide system and its role in shaping complex social interactions.

Researchers investigated the molecular evolution of the nonapeptide system in diverse Lake Tanganyika cichlid species. They performed a positive selection analysis using the dN/dS ratio to identify sites under evolutionary pressure. This was complemented by examining correlations between specific amino acid variants and two distinct social phenotypes observed in cichlids. Furthermore, gene expression data was analyzed to explore associations between brain receptor expression levels and variations in social phenotype, providing insights into both coding and regulatory changes within the nonapeptide system.

Analysis revealed that most sites within the nonapeptide receptors are under strong purifying selection, indicating their functional importance and conservation. However, a few specific sites, predominantly located in the extended intracellular loop 3 (IL3) of VTR2A receptors, exhibited clear signatures of positive selection, suggesting adaptive evolution in these regions.

A key finding was that a specific amino acid variant in the VTR2Aa receptor correlated directly with the pair-bonding social phenotype, strongly implying its potential involvement in the mechanisms underlying social attachment. Further gene expression analyses supported these findings, demonstrating that components of the nonapeptide system, including the VTR2Bb receptor and oxytocin (OT) itself, are differentially expressed across various social phenotypes. This differential expression highlights a significant role for regulatory variation, alongside the observed coding changes, in contributing to the diverse social behaviors seen in cichlids. These results collectively underscore how conserved neuroendocrine systems can adapt and diversify to support a wide range of social complexities.