Degeneration patterns of the olfactory receptor genes in sea snakes.

The sense of smell relies on the diversity of olfactory receptor (OR) repertoires in vertebrates. It has been hypothesized that different types of ORs are required in terrestrial and marine environments. Here we show that viviparous sea snakes, which do not rely on a terrestrial environment, have significantly lost ORs compared with their terrestrial relatives, supporting the hypothesis. On the other hand, oviparous sea snakes, which rely on a terrestrial environment for laying eggs, still maintain their ORs, reflecting the importance of the terrestrial environment for them. Furthermore, we found one Colubroidea snake (including sea snakes and their terrestrial relatives)-specific OR subfamily which had diverged widely during snake evolution after the blind snake-Colubroidea snake split. Interestingly, no pseudogenes are included in this subfamily in sea snakes, and this subfamily seems to have been expanding rapidly even in an underwater environment. These findings suggest that the Colubroidea-specific ORs detect nonvolatile odorants.