Amphibious Transitions Drive Lineage-Specific Diversification of Olfactory Receptors in Vertebrates
Olfactory receptors (ORs), which mediate chemical detection in vertebrates, constitute a highly dynamic and ecologically responsive gene family. While OR evolution has been well studied in fully terrestrial and aquatic lineages, its dynamics in amphibious vertebrates remain less explored. Species that occupy both aquatic and terrestrial habitats span a broad phylogenetic and ecological range--from amphibians to freshwater-dwelling mammals, semi-aquatic reptiles, and shoreline birds--and are subject to the functional demands of odour detection across two chemically disparate milieus. Here, we analysed OR gene repertoires across 230 vertebrate genomes, including 138 amphibious species. Our results show that OR repertoire expansion is not a uniform feature of terrestrial adaptation but is most pronounced in amphibious lineages, particularly those inhabiting freshwater systems, where chemically variable environments likely impose stronger selective pressures on olfaction. These expansions are primarily driven by lineage-specific expansions and correlate with ecologies that require sensing a wide range of chemical cues. While amphibious marine taxa possess larger OR repertoires than their fully marine relatives, they consistently exhibit fewer ORs than freshwater amphibious vertebrates. More broadly, species that rely on other sensory modalities--such as echolocation, electroreception, or vision--tend to exhibit reduced OR repertoires. Despite this diversity, several amphibious and terrestrial species within the same clade retain a small subset of shared OR genes, reflecting the retention of conserved OR orthologs--potentially those tuned to airborne odorants--across habitat transitions. However, overlap across clades is minimal, reflecting independent evolutionary responses to similar ecological pressures. Overall, our findings highlight amphibious lifestyles as key inflection points in vertebrate olfactory evolution--driving both OR repertoire expansion and divergence through the interplay of habitat complexity, sensory trade-offs, and lineage-specific constraints.