Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrhine primates

Abstract

Pheromones are water-soluble chemicals released and sensed by individuals of the same species to elicit social and reproductive behaviors or physiological changes; they are perceived primarily by the vomeronasal organ (VNO) in terrestrial vertebrates. Humans and some related primates possess only vestigial VNOs and have no or significantly reduced ability to detect pheromones, a phenomenon not well understood at the molecular level. Here we show that genes encoding the TRP2 ion channel and V1R pheromone receptors, two components of the vomeronasal pheromone signal transduction pathway, have been impaired and removed from functional constraints since shortly before the separation of hominoids and Old World monkeys Ϸ23 million years ago, and that the random inactivation of pheromone receptor genes is an ongoing process even in present-day humans. The phylogenetic distribution of vomeronasal pheromone insensitivity is concordant with those of conspicuous female sexual swelling and male trichromatic color vision, suggesting that a vision-based signaling-sensory mechanism may have in part replaced the VNO-mediated chemical-based system in the social͞reproductive activities of hominoids and Old World monkeys (catarrhines). P heromones are water-soluble chemicals used in intraspecific communications to elicit social and reproductive behaviors or physiological changes such as male-male aggression, onset of puberty, estrus, and induction of mating. Pheromones are perceived primarily by the vomeronasal organ (VNO), which is at the base of the nasal cavity and separated from the main olfactory epithelium that senses thousands of volatile odorants (1). It has been known for decades that some primate species, including humans, do not possess functional VNOs, and these organisms lack vomeronasal chemoreception to pheromones (1-3). This insensitivity has likely had important impacts on the sexual and social behaviors of many primates. On the other hand, behavioral changes may have also altered natural selection on vomeronasal chemoreception. It is therefore of interest to find when the vomeronasal pheromone insensitivity occurred in evolution, how it occurred, and why it occurred.Vomeronasal pheromone perception begins by the binding of pheromones to pheromone receptors located on the cell membrane of sensory neurons of the VNO, which triggers a signal transduction pathway that ultimately leads to the activation of the hypothalamus. Several components in the pathway have been identified, including GTP-binding proteins, phospholipase C, inositol 1,4,5-trisphosphate (IP3), and an ion channel of the transient receptor potential family named TRP2 (also known as TRPC2) (4). However, among members of this pathway, only TRP2 (5) and pheromone receptors of the V1R and V2R families (6-9) are unique to pheromone transduction and are not known to be used in other physiological processes. Disruption of either of these two components in mice hampers pheromone perception and causes dramatic changes in sexual and social behaviors (10-12). Th...

Comparative analysis of the role of olfaction and the neocortex in primate intrasexual competition

“…Although they have an intact accessory olfactory system, there is little evidence that it has a similar function in platyrrhines (with the exception of callitrichids). Catarrhines lack an intact AOS and show a significant reduction in olfactory behavior Meisami and Bhatnagar, 1998;Liman and Innan, 2003;Zhang and Webb, 2003).…”

“…Some platyrrhines are polymorphic for this trait: in several species, 50 -60% of females are trichromatic, while males are not (Jacobs, 1995;Jacobs and Deegan, 2001). As there is a concomitant reduction of the size, presence, and function of haplorhine olfactory structures, it has been argued that some of the functions of the strepsirrhine olfactory system may be served by the visual system in these taxa (Barton et al, 1995;Barton and Aggleton, 2000;Barton, 2001;Laska et al, 2003;Liman and Innan, 2003;Zhang and Webb, 2003;Gilad et al, 2004). Visual communication might be superior to olfaction because it allows conspecifics to identify individuals and assess social situations without coming into physical contact with odorants.…”

“…Webb et al (2004) tested this hypothesis in howling monkeys (Alouatta) by analyzing the gene encoding for an essential component of the VNO transduction pathway: the TRP2 ion channel. All catarrhines studied thus far have a TRP2 pseudogene rendering the VNO transduction pathway nonfunctional (Liman and Innan, 2003;Zhang and Webb, 2003). Webb et al (2004) found that in three species of Alouatta, the TRP2 is functional.…”

“…In fact, there are visual signals found in catarrhines that are not present in howling monkeys. Female sexual swellings are the most often cited example of evidence of an olfaction-vision trade-off (Barton and Aggleton, 2000; Barton, 2001;Liman and Innan, 2003;Zhang and Webb, 2003;Webb et al, 2004). Although reddening and slight swelling of genitalia can be found in all primate radiations, only Old World monkeys and apes have exaggerated swellings that can be seen from a distance.…”

Extraordinary Diversity of Chemosensory Receptor Gene Repertoires Among Vertebrates

“…In human, the entire V1R repertoire, including functional genes and pseudogenes, has approximately 200 members. However, the functional V1R repertoire is small, with only four open reading frames in most individuals Zhang and Webb 2003) . Grus et al (2005) identified functional V1R genes from five orders of placental and marsupial mammals ( Table 1 ).…”