Evolutionary dynamics and functional properties of chemoreceptors in ants

Chemoreceptors have been hypothesized to play essential roles in the origin and evolution of sociality in ants. In this study, we have performed a phylogenetic analysis of chemosensory genes in all seven currently available ant genomes. While the overall pattern of gene gain-and-loss was found to be similar in these species, we identified an exceptional expansion of a gustatory receptor subfamily in the red imported fire ant. Furthermore, signatures of positive selection were detected in multiple chemosensory gene subfamilies in all seven ant species, indicative of significant divergence in their chemosensory functions. We extended these analyses to a pair of non-social insect lineages (Drosophila and Anopheles) where abundant genomic resources are available; the comparison among these social and non-social insect lineages revealed several unique features of chemosensory gene evolution in ants.

In order to better understand the molecular mechanisms by which chemoreceptors regulate ant social behavior, we have cloned and heterologously expressed a number of odorant receptors (ORs) from a pair of highly divergent ant species, Harpegnathos saltator and Camponotus floridanus. Our results indicate that, in addition to the relatively high levels of sequence conservation observed between homologous ant ORs, there is also a surprising level of functional conservation between homologous ORs in these divergent species, with multiple instances of conserved ligand responses between orthologs. In addition to these heterologous studies, we have also started to examine the behavioral consequences of manipulating the olfactory systems of Camponotus workers using agonists and antagonists that specifically target the insect odorant coreceptor (Orco) protein. These manipulations have shown that the ant olfactory system is specifically involved in the recognition of non-nestmate individuals by workers.