Analysis of Immune Related Genes in Two Bumblebee Species Shows Decelerated Evolutionary Dynamics

     Recent studies have shown growing evidence for reduced immune system complexity in certain insect species, including social insects such as the honeybee Apis melifera, along with bumblebee species Bombus terrestris and Bombus impatiens. In contrast, Immune related genes generally evolve faster than corresponding non-immune genes. This rapid evolution is expected in colony-forming social insects, since colonies offer optimal conditions for various parasites. To further investigate this contradiction we sequenced 11 immune genes to assess the patterns of immune gene evolution in two populations each of two bumblebee species (Bombus terrestris and Bombus impatiens). We also sequenced four of these genes from four additional populations of B. impatiens. We found that the surveyed genes are generally conserved with minimal signals of elevated evolutionary dynamics. This could be caused by colony strategies creating an immune defense and/or high functional constraint at the sequenced regions of the genes. Notable exceptions were the antiviral RNAi genes Aubergine and Argonaute 2, which showed signals of rapid evolutionary dynamics. Even at alleged rapidly evolving immunologically important genes, our results demonstrate minimal divergence across populations and surprisingly slow adaptive evolution.