The evolution of animal chemosensory receptor gene repertoires: roles of chance and necessity

Chemosensory receptors recognize chemical signals of odorants and tastants in the environment. Six multigene families are known to encode chemosensory receptors in vertebrates and two multigene families in insects, respectively.Near-complete repertoires of chemosensory receptor genes have been determined from the genomic sequences of various organisms. These studies have shown that the number of chemosensory receptor genes is generally large and varies greatly among different organisms.A substantial portion of interspecific variation in gene number can be explained by adaptation to different environments. There was a large increase in the number of olfactory receptor genes when fishes evolved into land animals that needed to detect airborne odorants, but this number decreased drastically when mammals adapted to aquatic life.Vertebrate species have many chemosensory receptor pseudogenes as well as functional genes. There is no clear relationship between the number of pseudogenes and environmental changes.Genomic drift, a random process of gene duplication and deletion, has generated a large amount of gene copy-number variation among individuals of the same species. The number of odorant receptor genes and pseudogenes both followed a normal distribution in human populations.The similarity in the distribution of functional genes to the distribution of pseudogenes suggests that the copy-number variation of functional genes is almost neutral.The number of chemosensory receptor genes is generally much smaller in insects than in vertebrates. The proportion of pseudogenes is also smaller in insects.