Résumé
Olfactory receptors (ORs) located in the cell membrane of olfactory
sensory neurons of the nasal epithelium are responsible for odor
detection by binding specific odorant ligands. Primates are thought to
have a reduced sense of smell (microsmatic) with respect to other
mammals such as dogs or rodents. We have previously demonstrated that
over 70% of the human OR genes have become nonfunctional pseudogenes,
leading us to hypothesize that the reduced sense of smell could
correlate with the loss of functional genes. To extend these results,
we sampled the OR gene repertoire of 10 primate species, from prosimian
lemur to human, in addition to mouse. About 221 previously unidentified
primate sequences and 33 mouse sequences were analyzed. These sequences
encode ORs distributed in seven families and 56 subfamilies. Analysis
showed a high fraction (≈50% on average) of pseudogenes in
hominoids. In contrast, only ≈27% of OR genes are pseudogenes in Old
World monkeys, and New World monkeys are almost free of pseudogenes.
The prosimian branch seems to have evolved differently from the other
primates and has ≈37% pseudogene content. No pseudogenes were found
in mouse. With the exception of New World monkeys, we demonstrate that
primates have a high fraction of OR pseudogenes compared with mouse. We
hypothesize that under relaxed selective constraints, primates would
have progressively accumulated pseudogenes with the highest level seen
in hominoids. The fraction of pseudogenes in the OR gene repertoire
could parallel the evolution of the olfactory sensory function.