A Unique Expansion of the UV-Violet Visual Opsin Palette in a Group of Freshwater Snakes

Snakes are known to express up to three visual pigments, a rhodopsin and two cone opsins, SWS1 and LWS. Colubrid snakes in the genus Helicops have invaded freshwater habitats from an ancestral terrestrial-only condition. Here we report, for the first time in a tetrapod, the presence of two distinct SWS1 opsins simultaneously expressed in different photoreceptors, conferring both UV and violet sensitivity in a Helicops species. Molecular analysis and in vitro expression confirm the presence of two functional SWS1 copies, likely the result of a recent gene duplication. Evolutionary analyses indicate that each sws1 has undergone different evolutionary paths, with strong purifying selection acting on the UV-sensitive copy and neutrality on the violet-sensitive copy. Site-directed mutagenesis shows, for the first time in snakes, the functional role of a single amino acid substitution, Phe86Val, in the large spectral shift between UV and violet opsins. In addition, higher densities of photoreceptors and SWS cones in the ventral retina suggest improved acuity in the upper visual field possibly correlated with visually-guided behaviors. The expanded visual opsin repertoire and the specialized retinal architecture are likely to improve photon uptake in underwater and terrestrial environments, and provide the neural substrate for a gain in chromatic discrimination, potentially conferring unique color vision in the UV-violet range. Our findings highlight the innovative mechanisms undertaken by a highly specialized tetrapod lineage to tackle the challenges imposed by the invasion of novel photic environments and the extraordinary diversity of evolutionary trajectories taken by visual opsin- based perception in vertebrates.