Remarkable diversity of mammalian pheromone receptor repertoires

At the center of animal species survival is the ability of individuals to identify members of their own species and to discriminate between the genders of these members to procreate. This basic biological task is, in mammals, mostly mediated by the exchange of pheromonal information and performed by the olfactory system. More precisely, an elongated tubular structure, the vomeronasal organ, located in the nasal cavity and filled with sensory neurons, is mainly responsible for the detection of intraspecies chemosensory signals (1). Receptors responsible for the recognition of pheromones (2, 3) and expressed by vomeronasal sensory neurons are G protein-coupled receptors, termed V1r receptors (4). The genes encoding these latter are particularly numerous in rodents (>100) and form a very diverse superfamily (5–8). In a recent issue of PNAS, Grus et al. (9) reported the identification of the V1r pheromone receptor repertoires pertaining to multiple orders of marsupial and placental mammals, and they observed striking variations in terms of repertoire size and content between these orders. Such extent of variability in the mammalian class for a given gene family is unusual, to say the least, and forces us to reflect on the nature of the evolutionary forces that led to such unequally distributed chemosensory tools among mammals.

Mammals constitute a large group, which includes monotremes, marsupials, and eutherians, these latter comprising species apparently as unrelated as bats and whales. Our current view of the molecular chemosensory tools (i.e., the chemosensory receptor gene repertoires) available to the different mammalian species is extremely …