Priority effects at the strain level in honeybee gut microbiota

Gut microbial communities often differ at the strain level even among closely related individuals, but the ecological mechanisms driving this variation remain poorly understood. One potential driver is priority effects, differences in the timing and order of microbial colonization, which can lead to the assembly of distinct communities, even under similar environmental conditions. Priority effects may specifically play an important role in shaping microbial communities at the strain level, given that strains of the same species typically occupy similar ecological niches. To test this, we examined gut microbiota assembly in honeybees, in which age-matched nestmates are known to host similar microbial communities at the species level but vary in strain composition. We sequentially colonized microbiota-depleted honeybees with two distinct microbial communities, each composed of the same twelve species but different strains. We found that firstcomer strains consistently dominated the resulting communities, though the strength of these priority effects varied among closely related strains and species. Dropping out individual strains from the firstcomer community only partially improved the colonization success of latecomer conspecifics, suggesting that priority effects also act across species boundaries. Our results underscore the importance of priority effects for gut microbial community assembly at the strain level and in shaping the specialized gut microbiota of bees.