Neotropical bee microbiomes pointto a fragmented social core and strongspecies‑level effects

Background:
Individuals that band together create new ecological opportunities for microorganisms. In vertical transmission, theory predicts a conserved microbiota within lineages, especially social bees. Bees exhibit solitary to social behavior among and/or within species, while life cycles can be annual or perennial. Bee nests may be used over generations or only once, and foraging ecology varies widely. To assess which traits are associated with bee microbiomes, we analyzed microbial diversity within solitary and social bees of Apidae, Colletidae, and Halictidae, three bee families in Panama’s tropical forests. Our analysis considered the microbiome of adult gut contents repli- cated through time, localities, and seasons (wet and dry) and included bee morphology and comparison to abdomi- nal (dissected) microbiota. Diversity and distribution of tropical bee microbes (TBM) within the corbiculate bee clade were emphasized.
Results:
We found the eusocial corbiculate bees tended to possess a more conserved gut microbiome, attribut- able to vertical transmission, but microbial composition varied among closely related species. Euglossine bees (or orchid bees), corbiculates with mainly solitary behavior, had more variable gut microbiomes. Their shorter-tongued and highly seasonal species displayed greater diversity, attributable to flower-visiting habits. Surprisingly, many sting- less bees, the oldest corbiculate clade, lacked bacterial genera thought to predate eusociality, while several faculta- tively social, and solitary bee species possessed those bacterial taxa. Indeed, nearly all bee species displayed a range of affinities for single or multiple variants of the “socially associated” bacterial taxa, which unexpectedly demonstrated high sequence variation.
Conclusions:
Taken together, these results call into question whether specific bacterial associates facilitate eusocial behavior, or are subsequently adopted, or indicate frequent horizontal transmission between perennial eusocial colo- nies and other social, facultatively social, and solitary bees.