Chemical communication is fundamental for the operation of insect societies. Their diverse vocabulary of chemical signals requires a correspondingly diverse set of chemosensory receptors. Insect olfactory receptors (ORs) are the largest family of chemosensory receptors. The OR family is characterized by frequent expansions of subfamilies, in which duplicated ORs may adapt to detect new signals through positive selection on their amino acid sequence. Ants are an extreme example with ~400 ORs per genome-the highest number in insects. Presumably, this reflects an increased complexity of chemical communication. Here, we examined gene duplications and positive selection on ant ORs. We reconstructed the hymenopteran OR gene tree, including five ant species, and inferred positive selection along every branch using the branch-site test, a total of 3326 tests. We find more positive selection in branches following species-specific duplications. We identified amino acid sites targeted by positive selection, and mapped them onto a structural model of insect ORs. Seventeen sites were under positive selection in six or more branches, forming two clusters on the extracellular side of the receptor, on either side of a cleft in the structure. This region was previously implicated in ligand activation, suggesting that the concentration of positively selected sites in this region is related to adaptive evolution of ligand binding sites or allosteric transmission of ligand activation. These results provide insights into the specific OR subfamilies and individual residues that facilitated adaptive evolution of olfactory functions, potentially explaining the elaboration of chemical signaling in ant societies.
Bibliographical noteFunding Information:
All computations were conducted on the Hive computer cluster of the Faculty of Natural Science at the University of Haifa. E.P. was supported by grants from the Israel Science Foundation (grant numbers 646/15, 2140/15, 2155/15) and the US-Israel Binational Science Foundation (grant 2013408). M.K. was supported by grants from the Israel Science Foundation (grant numbers 1454/13, 1959/13, 2155/15)
© The Author(s) 2018.
- Gene duplication
- Olfactory receptors
- Positive selection
- Protein structure
- Social insects
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics