Visual adaptation of opsin genes to the aquatic environment in sea snakes
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RESEARCH ARTICLE
Open Access
Visual adaptation of opsin genes to the aquatic environment in sea snakes Takashi Seiko1, Takushi Kishida2, Mina Toyama3, Takahiko Hariyama3, Takashi Okitsu4, Akimori Wada4, Mamoru Toda5, Yoko Satta1 and Yohey Terai1*
Abstract Background: Evolutionary transitions from terrestrial to aquatic life history cause drastic changes in sensory systems. Indeed, the drastic changes in vision have been reported in many aquatic amniotes, convergently. Recently, the opsin genes of the full-aquatic sea snakes have been reported. However, those of the amphibious sea snakes have not been examined in detail. Results: Here, we investigated opsin genes and visual pigments of sea snakes. We determined the sequences of SWS1, LWS, and RH1 genes from one terrestrial, three amphibious and four fully-aquatic elapids. Amino acid replacements at four and one spectra-tuning positions were found in LWS and RH1, respectively. We measured or predicted absorption of LWS and RH1 pigments with A1-derived retinal. During their evolution, blue shifts of LWS pigments have occurred stepwise in amphibious sea snakes and convergently in both amphibious and fully-aquatic species. Conclusions: Blue shifted LWS pigments may have adapted to deep water or open water environments dominated by blue light. The evolution of opsins differs between marine mammals (cetaceans and pinnipeds) and sea snakes in two fundamental ways: (1) pseudogenization of opsins in marine mammals; and (2) large blue shifts of LWS pigments in sea snakes. It may be possible to explain these two differences at the level of photoreceptor cell composition given that cone and rod cells both exist in mammals whereas only cone cells exist in fully-aquatic sea snakes. We hypothesize that the differences in photoreceptor cell compositions may have differentially affected the evolution of opsins in divergent amniote lineages. Keywords: Visual adaptation, Opsin genes, Sea snakes, Visual pigments, Aquatic amniotes Background Among amniotes, several major extant taxonomic groups have returned to the sea and have adapted to an aquatic environment. The best-known group is cetacea, composed of toothed and baleen whales. Sirenia is a group that includes the dugong and manatee which both inhabit relatively shallow water. Among reptiles, hydrophiin sea snakes have also adapted to an aquatic environment *Correspondence: [email protected] 1 Department of Evolutionary Studies of Biosystems, SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, Kanagawa 240‑0193, Japan Full list of author information is available at the end of the article
completely, and do not rely on terrestrial habitat anymore [1, 2]. Species in these three amniote groups are fully aquatic, give birth to live young, and spend their whole life in water. Sea snakes are composed of species in the fully aquatic hydrophiins and amphibious laticaudins (also called sea kraits) [3]. Molecular phylogenetic studies have reported that Hydrophiini forms a monophyletic clade with ter
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