Signatures of selection in pleiotropic genes involved in insect neuronal and immune systems.

Pleiotropy, where a single gene contributes to multiple biological functions, plays a central role in shaping evolutionary constraints. The nervous and immune systems are tightly integrated both functionally and genetically, but it is not clear whether pleiotropy constrains adaptation in each system or contributes to disease. Building on evidence that genes that are pleiotropic between development and immunity in Drosophila melanogaster evolve slower than immune-specific genes, we tested whether pleiotropic neuro-immune genes likewise evolve under stronger purifying selection compared to genes functioning solely in neuronal or immune processes, and whether such evolutionary constraint is associated with human neurological disease. We identified immune, neuronal, and neuro-immune genes in D. melanogaster using Gene Ontology annotations, estimated patterns of evolutionary constraint across 12 Drosophila species using dN/dS ratios, and investigated the breadth of their expression across developmental stages as a complementary proxy for pleiotropy. We further curated associations between the human orthologs for these genes and neurological disease and tested whether their evolutionary statistics and pleiotropic status predicted disease involvement. We found that pleiotropic genes exhibited significantly lower dN/dS values and were less stage-specific than non-pleiotropic immune genes. Slower-evolving genes were more likely to be associated with human neurological diseases but this pattern was strongest for non-pleiotropic neuronal genes, suggesting that pleiotropy alone is not the strongest predictor of disease. Our results therefore indicate that dN/dS could be a stronger predictor of disease association than pleiotropy, raising new questions about the maintenance of pleiotropy in evolutionarily dynamic physiological systems.