Functional phylosymbiosis in birds: phylogeny and antimicrobial properties of bacterial symbionts in Bucerotiformes

Bacterial symbionts and their animal hosts differ in the strength and type of selection pressures that promote their evolution and, as some host traits are phylogenetically conserved, their symbiotic bacterial communities could also exhibit greater similarity than expected by chance; a pattern known as phylosymbiosis. This might result from differences in bacterial assemblages across species-specific habitats, and/or from natural selection favouring host traits that allow harbouring functional species-specific bacteria (functional phylosymbiosis). We explored these patterns in eight Bucerotiformes taxa, including hoopoes, woodhoopoes, and hornbills, of which hoopoes and woodhoopoes harbour antibiotic-producing symbionts in their uropygial gland. We characterised uropygial secretion bacterial communities using high-throughput 16S-rRNA gene sequencing and analysed its association with avian phylogeny. Furthermore, we quantified antimicrobial activity of bacterial isolates, and explored its relationship with host phylogenetic distances and bacterial community composition. We found a significant correlation between avian phylogeny and bacterial community dissimilarities, supporting phylosymbiosis. Moreover, antimicrobial activity was significantly predicted by bacterial community composition and the relative abundance of particular strains in hoopoes and woodhoopoes. These findings suggest that antimicrobial properties of symbionts could be responsible for the detected phylosymbiosis and that natural selection may favour uropygial gland features allowing hosts to harbour specific antimicrobial-producing symbionts.