Gain and loss of gut symbionts during stingless bee diversification is linked to host group size

Abstract Microbiomes typically vary between closely related host species, but how and why they diverge during host diversification remain poorly understood. Bees provide a well-established model for the role of sociality in microbiome evolution: solitary species have generalist gut bacterial symbionts, while eusocial honey bees and bumble bees have host-specific “core” gut bacterial symbionts. Curiously however, some species of stingless bees—the most diverse clade of eusocial bees—lack the core symbionts Snodgrassella and Gilliamella . Here, we tested whether group size, a fundamental trait of social organisms, explains symbiont gain and loss in stingless bees. Using specimens from a global collection, we performed 16S rRNA gene sequencing and qPCR on 187 adult workers representing 29 species and 18 genera. Maximum reported colony sizes for these species range from ∼100 to over ∼100,000 individuals. Our phylogenetic models indicate positive feedbacks between group size and the symbionts Snodgrassella and Gilliamella . We suggest that challenges faced by large groups (e.g., disease risk) may select for symbionts that supply needed functionality (e.g., pathogen protection). Our findings highlight group size as a potentially widespread driver of microbiome evolution in social hosts.