The olfactory receptor gene family of marsupials

Olfaction in vertebrates is mediated mainly by a large family of olfactory receptors in the olfactory epithelium that belong to the superfamily of G protein-coupled receptors. Olfactory systems are well conserved among vertebrates, including marsupials, but there is a large variation in the numbers of olfactory genes in different animals. Most marsupials are nocturnal so depend on their sense of smell to locate food, avoid predators and identify potential mates in similar ways to other mammals. The olfactory bulbs are quite large in adult marsupials, suggesting that the sense of smell is very important in these animals. In addition, very undeveloped newborn marsupials have the special challenge of locating the pouch unassisted. It is likely that these newborns utilise their sense of smell for this unique pouch-finding task. The olfactory system is one of the few systems that is sufficiently developed in newborn marsupials to accomplish the task of finding the pouch. The opossum OR repertoire of one marsupial, the American opossum, is one of the largest characterised in mammals so far, containing over 1,500 genes. Interestingly comparisons of the opossum OR repertoire with the repertoire in an Australian marsupial, the tammar wallaby, suggests that a large conserved OR repertoire may be a feature of marsupials. The OR repertoires of the two marsupials show a high degree of similarity in total gene numbers and range of genes. This is unlike placental mammals, where the OR repertoires show a greater range. Results from these comparisons provide evidence for both the major forces (adaptation and genomic drift) behind the birth-and-death theory for the evolution of OR genes.