Comparative genomics sheds light on the evolutionary consequences of pollen specialization in the sensory genes of bees

The dietary breadth of bee species varies widely: while most species collect pollen from a broad range of floral resources (hereafter, generalists), few restrict their pollen collection to a narrow subset of available floral resources (hereafter, specialists). Despite this wide variation in diet breadth among bees, the evolutionary consequences of transitions from generalist to specialist diets remain largely unknown. In this study, we hypothesize that changes in the abundance and diversity of floral resources used by specialist bees shape the evolution of their olfactory systems. We test this hypothesis by investigating patterns of molecular evolution among three families of olfactory receptor genes (odorant receptors, ionotropic receptors, and gustatory receptors) between generalist and specialist bees. We generated de-novo genomes for six species of generalist and specialist bees in the genera Eucera and Melissodes (Hymenoptera: Apidae: Eucerini). Combining this novel dataset with 49 previously sequenced genomes across the bee clade Anthophila, we assess the relationship between dietary breadth and evolutionary patterns of olfactory receptor genes. This study furthers our understanding of the underlying evolutionary mechanisms of insect pollen specialization.