Genomic drift with purifying selection that acts only on a subset of the genes and a combination of balancing and perhaps also positive selection result in genetically highly diverse chemosensory receptors. What are the consequences of this genetic variability? In humans, genetic diversity will result in perceptual diversity. Each individual perceives olfactory stimuli with their personal set of ORs. Olender et al
] identified 244 ORs for which both an intact and a pseudogenized version are found in the population. In two randomly selected individuals one third of OR alleles will be functional in one individual but non-functional in the other individual. These huge differences in the sensory apparatus are very likely to have dramatic influences on how odorous stimuli are perceived and it has already been shown that, in some cases, genetic variability in ORs influences how strong and pleasant the ligands of the ORs are perceived to be [6
Similar to the findings in human ORs, Wynn et al
] found that, for a large percentage of mouse VR genes, both an intact version and a version that is truncated or carries a frame shift mutation are present. The repertoire of functional VRs with which different mouse strains perceive stimuli therefore varies significantly. Because mouse VRs sense chemical signals that are used in social communication and often have pheromonal activity [7
], this variability in VR genes may result in variability in pheromone-mediated behaviors. Differences in such behaviors have been observed between the mouse strains that were analyzed by Wynn et al
] and it is tempting to speculate that the genetic differences in the VRs contribute to these behavioral differences. However, to fully understand the relationship between genetic VR variation and behavioral variability, the ligands that bind to the VRs have to be identified.
Although for most VRs the ligands are unknown, groups of V2Rs that respond preferentially to conspecific odor cues, odors from other rodent species and subspecies, or predator odors have recently been identified [8
]. Interestingly, Wynn et al
] found more genetic variability in V2Rs that are tuned to olfactory signals from other mouse subspecies than in receptors tuned to the smell of predators. Based on this finding the authors speculate that genetic variability in V2Rs could be a driving force of speciation by mediating a behavioral barrier that contributes to reproductive isolation between subspecies. A similar process has been described in moths, where genetic variability in pheromone receptors contributes to speciation [9
]. Rather than being inconsequential noise in highly redundant genes, chemosensory receptor gene variability may in some cases be driving sympatric speciation.
We are only just beginning to understand the causes and consequences of the unusual genetic and functional variability of large chemosensory receptor gene repertoires in different species. The comprehensive data on genetic variability in the human OR and mouse VR repertoire presented in the articles by Olender et al
] and Wynn et al
] will be invaluable in tackling these questions.