Rodent Urinary Proteins: Genetic Identity Signals and Pheromones

Abstract

Many rodents communicate through scent marks that contain a complex mixture of volatile components along with species-specific proteins of the lipocalin family. These proteins have an internal cavity that binds low molecular weight hydrophobic volatiles and can slow their release from scent marks, while the proteins themselves are detected through receptors in the vomeronasal organ. By far the best studied urinary lipocalins to date are the major urinary proteins (MUPs) of the house mouse (Mus musculus domesticus). MUPs are encoded by a cluster of >21 genes on mouse chromosome 4 that has undergone recent rapid expansion in this species. Mice of both sexes make substantial investment in these proteins, with considerable polymorphism in the patterns of MUPs expressed between individuals. This MUP pattern provides the main genetic identity signal in mouse urine scent used to recognise individual scent owners, close kin, and can also be used to assess genetic heterozygosity. A male-specific atypical MUP named darcin acts as a sex pheromone, responsible for eliciting instinctive female sexual attraction to spend time near male urinary scent marks. Importantly, darcin also stimulates rapid associative learning, such that females that contact darcin learn and are subsequently attracted to the associated airborne odour of that particular male but not to airborne odours of other males. This pheromone-induced mechanism of learned attraction targets and reinforces sexual attraction towards a particular male, allowing sexual attraction to males to be inherent but also selective. Comparative studies of other rodents are starting to reveal considerable diversity in concentration, sexual ­dimorphism and polymorphic complexity of lipocalins in scent signals; these differences are likely to reflect species-specific requirements for recognition and ­assessment through scent.