Strain diversity and host specificity in a specialized gut symbiont of honeybees and bumblebees.

Host-restricted lineages of gut bacteria often include many closely related strains, but this fine-scale diversity is rarely investigated. The specialized gut symbiont Snodgrassella alvi has codiversified with honeybees (Apis mellifera) and bumblebees (Bombus) for millions of years. Snodgrassella alvi strains are nearly identical for 16S rRNA gene sequences but have distinct gene repertoires potentially affecting host biology and community interactions. We examined S. alvi strain diversity within and between hosts using deep sequencing both of a single-copy coding gene (minD) and of the V4 region of the 16S rRNA gene. We sampled workers from domestic and feral A. mellifera colonies and wild-caught Bombus representing 14 species. Conventional analyses of community profiles, based on the V4 region of the 16S rRNA gene, failed to expose most strain variation. In contrast, the minD analysis revealed extensive strain variation within and between host species and individuals. Snodgrassella alvi strain diversity is significantly higher in A. mellifera than in Bombus, supporting the hypothesis that colony founding by swarms of workers enables retention of more diversity than colony founding by a single queen. Most Bombus individuals (72%) are dominated by a single S. alvi strain, whereas most A. mellifera (86%) possess multiple strains. No S. alvi strains are shared between A. mellifera and Bombus, indicating some host specificity. Among Bombus-restricted strains, some are restricted to a single host species or subgenus, while others occur in multiple subgenera. Findings demonstrate that strains diversify both within and between host species and can be highly specific or relatively generalized in their host associations.