Microbial gut symbiont communities of native Bombus differ by species and not by habitat type

Background/Question/Methods

Bacterial symbionts of insects can have important functions in defense, disease, and nutrition. However little is known as to how microbial communities vary among individuals within and between populations, or how they respond to anthropogenic disturbances. Some species of bumble bees (Genus Bombus) have experienced dramatic declines in the past few decades. While bumble bee guts are known to contain distinct microbial gut symbionts, no studies have reported high-throughput sequencing examinations of symbiont communities in this genus. We collected three species of native bumble bees (Bombus bimaculatus, B. griseocollis and B. impatiens) at six active cranberry farms and six semi-natural bogs in southern New Jersey, USA. To assess the bacterial gut communities, we sequenced amplifications of the 16S rRNA using 454 pyrosequencing. The objectives of this study were to determine whether microbial gut symbiont communities differ by 1) semi-natural and agricultural habitats, and/or 2) bee species, and to assess individual variation in community size and composition.

Results/Conclusions

We sequenced 100 individual bees and found total of 352 operational taxonomic units (OTUs) with only 76 OTUs representing approximately 99% of all reads. The symbiont communities, differed statistically between semi-natural and agricultural habitats (F=2.14, DF=1, P=0.03), but this explained little of the variation (R2=0.02). Similarly, beta diversity as calculated across sites did not differ between farm and abandoned bog. We found greater differences in symbiont communities among bee species (F=17.21, DF=2, P<0.001, R2=0.26). While B. bimaculatus had the greatest diversity of bacterial OTUs, many of these were environmental associates, including a number of plant-associated bacteria, and likely not specific to bumble bees; also, B. bimaculatus gut communities were smaller. The B. griseocollis and B. impatiens symbiont communities were larger and were dominated by bacterial phylotypes such as Gilliamella (Gammaproteobacteria) and Snodgrassella (Betaproteobacteria). These are known as distinctive associates of honey bees and bumble bees, and are associated with parasite resistance in studies of European bumble bees. Overall, our results suggest that gut symbiont communities vary more across host bee species than they do across a land-use gradient. This species-level variation may have important functional consequences for bumble bees that do or do not harbor certain bacterial phylotypes.