Vast differences in strain-level diversity in the gut microbiota of two closely related honey bee species

Most bacterial species encompass strains with vastly different gene content. Strain diversity in microbial communities is therefore considered to be of functional importance. Yet, little is known about the extent to which related microbial communities differ in diversity at this level and which underlying mechanisms may constrain and maintain strain-level diversity. Here, we used shotgun metagenomics to characterize and compare the gut microbiota of two honey bee species, Apis mellifera and Apis cerana, which have diverged about 6 mio years ago. While both host species are colonized by largely overlapping bacterial 16S rRNA phylotypes, we find that their communities are highly host-specific when analyzed with genomic resolution. Despite their similar ecology, A. mellifera displayed a much higher extent of strain-level diversity and functional gene content in the microbiota than A. cerana, per colony and per individual bee. In particular, the gene repertoire for polysaccharide degradation was massively expanded in the microbiota of A. mellifera relative to A. cerana . Bee management practices, divergent ecological adaptation, or habitat size may have contributed to the observed differences in microbiota composition of these two key pollinator species. Our results illustrate that the gut microbiota of closely related animal hosts can differ vastly in genomic diversity despite sharing similar levels of diversity at the 16S rRNA gene. This is likely to have consequences for gut microbiota functioning and host-symbiont interactions, highlighting the need for metagenomic studies to understand the ecology and evolution of microbial communities.