[Massive gene losses: the case of chemoreceptors in aquatic tetrapods]

The existence of multigene families is the result of gene duplication. In vertebrates, the genes coding for the chemoreceptors involved in olfaction often form large families, sometimes comprising several thousand genes. In this case, it is assumed that a large number of genes is essential to discriminate between a large number of odorant molecules, and that the ability to identify a large number of odors may be necessary for different purposes, such as finding food and sexual partners, and avoiding predators. However, it has been found that in some species the number of these genes is very small, resulting from the secondary loss of many genes. This massive loss of genes is not always clearly associated with a change in the biology of the species or its environment, but in some cases, it is associated with drastic changes, such as the return to aquatic life in tetrapods.