Evolution of the immune system of bees: A story of conservation, gains, and losses

Background Insect pollinators, such as bees, provide essential ecosystem services yet face increasing environmental challenges, including pathogens, which can negatively impact host fitness. Central to host defences are immune genes and their products but for many bee species, our understanding of the conservation of the immune gene repertoire, as well as mechanisms that allow for functional diversity, is restricted to a few species. Results Here, we perform a pan-clade examination of the canonical immune genes and associated functional gene groups found across 70 bee species, representing six of the seven extant families. We show a high level of conservation of immune genes with all major immune gene groups represented, with elevated copy number variation found in CLIP-domain serine proteases, serpins, and short repeat regulatory proteins across bee species. Using the buff-tailed bumblebee Bombus terrestris , we further show that increases in immune-related gene group size are generally associated with increased nucleotide diversity, and transcriptional divergence among group members suggestive that group size, through mechanisms such as gene duplication, may allow for structural and functional diversity across immune components. However, we find that this pattern is non-linear indicating that gene group expansion is constrained. We also find lineage-specific losses of antimicrobial peptides highlighting that certain immune components may be dispensable or compensated by other elements. Conclusions Our analyses show that the genetic components of bee immunity are largely conserved, with duplication and loss highlighted as mechanisms that shape immune diversity, which, collectively, has implications for understanding resilience of bee species to increasing pathogenic threats.