Comparative transcriptomics and the evolution of sociality in bees

A major challenge in biology is to understand the evolution of social organization in animals. A promising avenue toward this goal is the comparative evolutionary analysis of animal species with varying degrees of sociality. We take this approach using molecular evolution techniques aimed at identifying adaptive genetic changes associated with social evolution among a group of socially diverse bee species. We generated brain expressed sequence tags (ESTs) for 12 bee species exhibiting a wide diversity of social structure, including multiple members of the four corbiculate bee tribes (honey bees, orchid bees, bumble bees, and stingless bees). ESTs were combined with data from the sequenced honey bee (Apis mellifera) genome to create multiple sequence alignments of orthologous coding regions. We fitted these alignments by maximum likelihood to codon substitution models to search for molecular signatures of positive selection specific to the social or solitary lineages. A large number of genes showing significantly different rates of evolution between the groups were identified, including a subset under positive selection. Many genes identified as evolving more rapidly in social relative to solitary lineages fall into several Gene Ontology categories related to behavior, including learning and memory, brain morphogenesis, and olfactory behavior. The results of this comparative transcriptomics study shed light on adaptive genetic changes and mechanistic processes that may have been involved in bee social evolution.