Function and Evolution of Putative Odorant Carriers in the Honey Bee (Apis mellifera)

Abstract

The remarkable olfactory power of insect species is thought to be generated by a combinatorial action of G-protein-coupled olfactory receptors (ORs) and olfactory carriers. Two such carrier gene families are found in insects: the odorant binding proteins (OBPs) and the chemosensory proteins (CSPs). In olfactory sensilla, OBPs and CSPs are believed to deliver hydrophobic airborne molecules to ORs, but their expression in non-olfactory tissues suggests that they also may function as general carriers in other developmental and physiological processes. Bioinformatics and experimental approaches were used to characterise the OBP and CSP gene families in a highly social insect, the western honey bee (Apis mellifera). Comparison with other insects reveals that the honey bee has the smallest set of these genes, consisting of only 21 OBPs and 6 CSPs. These numbers stand in stark contrast to the 66 OBPs and 7 CSPs in the mosquito Anopheles gambiae and the 46 OBPs and 20 CSPs in the beetle Tribolium castaneum. The genes belonging to both families are often organised in clusters, and evolve by lineage speci c expansions. Positive selection has been found to play a role in generating a greater sequence diversi cation