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

Microbiomes typically vary between closely related host species, but how and why they diverge during host diversification remain poorly understood. In bees, the evolution of eusociality is thought to have promoted symbiosis with specialized gut bacteria, including Snodgrassella and Gilliamella. While these symbionts are common in bumble bees and honey bees, some stingless bees, a closely related clade of eusocial bees, lack them. Here, we tested whether group size, a fundamental trait of social organisms with implications for microbial transmission and function, explains symbiont gain and loss in stingless bees. Using specimens from a global (though mostly Neotropical) collection, we performed 16S rRNA gene sequencing and qPCR on adult worker bees representing 29 species from 18 genera. Phylogenetic models indicate that bee species with larger colonies are more likely to harbor Snodgrassella and Gilliamella. We propose two underlying mechanisms: challenges faced by large groups (e.g., disease risk) select for symbionts that supply needed functionality (e.g., pathogen protection), or large groups are less prone to transmission bottlenecks and stochastic symbiont loss. While deeper sampling within species is still needed, these findings highlight group size as a driver of microbiome evolution in stingless bees and, potentially, in other social hosts.