Evolutionary history of tyrosine-supplementing endosymbionts in pollen-feeding Dasytinae (Coleoptera: Melyridae) beetles

Mutualistic relationships with bacteria can facilitate the colonization of ecological niches. However, detailed comparative studies of symbiont presence and function across host taxa are needed to unravel the evolutionary origins and dynamics of symbioses and their contributions to host fitness. Here, we investigated the evolutionary history and function of the symbiosis between pollen-feeding soft-winged flower beetles (Melyridae, Dasytinae) and their intracellular gamma-proteobacterial partner ‘Candidatus Dasytiphilus stammeri’. Reconstructing the distribution of the symbiont within the Dasytinae phylogeny unraveled a long-term coevolution, originating from a single acquisition event with subsequent host-symbiont codiversification, but also several independent symbiont losses. The analysis of 20 different symbiont genomes revealed that their genomes are severely eroded. However, the universally retained shikimate pathway indicates that the core metabolic contribution to their hosts is the provision of tyrosine for cuticle sclerotization and melanization. Despite the high degree of similarity in gene content and order across symbiont strains, the capacity to synthesize additional essential amino acids and vitamins and to recycle urea is retained in some but not all symbionts, suggesting ecological differences among host lineages. This report of tyrosine-provisioning symbionts in pollen-feeding insects expands our understanding of tyrosine supplementation as an important symbiont-provided benefit across a broad range of insects with diverse feeding ecologies.