Functional Evolution of Mammalian Odorant Receptors

Abstract

The mammalian odorant receptor (OR) repertoire is an attractive model to study evolution, because ORs have been subjected to rapid evolution between species, presumably caused by changes of the olfactory system to adapt to the environment. However, functional assessment of ORs in related species remains largely untested. Here we investigated the functional properties of primate and rodent ORs to determine how well evolutionary distance predicts functional characteristics. Using human and mouse ORs with previously identified ligands, we cloned 18 OR orthologs from chimpanzee and rhesus macaque and 17 mouse-rat orthologous pairs that are broadly representative of the OR repertoire. We functionally characterized the in vitro responses of ORs to a wide panel of odors and found similar ligand selectivity but dramatic differences in response magnitude. 87% of human-primate orthologs and 94% of mouse-rat orthologs showed differences in receptor potency (EC50) and/or efficacy (dynamic range) to an individual ligand. Notably dN/dS ratio, an indication of selective pressure during evolution, does not predict functional similarities between orthologs. Additionally, we found that orthologs responded to a common ligand 82% of the time, while human OR paralogs of the same subfamily responded to the common ligand only 33% of the time. Our results suggest that, while OR orthologs tend to show conserved ligand selectivity, their potency and/or efficacy dynamically change during evolution, even in closely related species. These functional changes in orthologs provide a platform for examining how the evolution of ORs can meet species-specific demands.

Author Summary

The mammalian odorant receptor repertoire has been subjected to significant gene duplication and gene loss between species, presumably to adapt to the environment of an organism. However, even in distantly related species, a clear orthologous relationship exists for many genes. While ligands have been identified for several ORs, many of these receptors remain uncharacterized, especially in species other than human and mouse. Due to this paucity of functional data, it is assumed that ORs with similar sequence share functional characteristics. Here we investigate the functional evolution of OR orthologs—genes related via speciation—and OR paralogs—genes related via a duplication event—to provide insight as to how this large gene family has evolved. We show that OR orthologs have similar ligand selectivity to a panel of odors but differ in response magnitude. Additionally, orthologs respond to a common ligand more often than human OR paralogs, but there are vast differences in the potency and efficacy of individual receptors. This result stresses the broad importance of combining evolutionary genomics and molecular biology approaches to study gene function.