title: Visual system evolution and the nature of the ancestral snake.
abstract: The dominant hypothesis for the evolutionary origin of snakes from 'lizards' (non-snake squamates) is that stem snakes acquired many snake features while passing through a profound burrowing (fossorial) phase. To investigate this, we examined the visual pigments and their encoding opsin genes in a range of squamate reptiles, focusing on fossorial lizards and snakes. We sequenced opsin transcripts isolated from retinal cDNA and used microspectrophotometry to measure directly the spectral absorbance of the photoreceptor visual pigments in a subset of samples. In snakes, but not lizards, dedicated fossoriality (as in Scolecophidia and the alethinophidian Anilius scytale) corresponds with loss of all visual opsins other than RH1 (λmax 490-497 nm); all other snakes (including less dedicated burrowers) also have functional sws1 and lws opsin genes. In contrast, the retinas of all lizards sampled, even highly fossorial amphisbaenians with reduced eyes, express functional lws, sws1, sws2 and rh1 genes, and most also express rh2 (i.e. they express all five of the visual opsin genes present in the ancestral vertebrate). Our evidence of visual pigment complements suggests that the visual system of stem snakes was partly reduced, with two (RH2 and SWS2) of the ancestral vertebrate visual pigments being eliminated, but that this did not extend to the extreme additional loss of SWS1 and LWS that subsequently occurred (probably independently) in highly fossorial extant scolecophidians and A. scytale. We therefore consider it unlikely that the ancestral snake was as fossorial as extant scolecophidians, whether or not the latter are para- or monophyletic.
authors.affiliation: Department of Life Sciences, The Natural History Museum, London, UK.
authors.forename: B F
authors.initials: BF
authors.lastname: Simões
authors.affiliation: Department of Life Sciences, The Natural History Museum, London, UK.
authors.forename: F L
authors.initials: FL
authors.lastname: Sampaio
authors.affiliation: Laboratório de Biologia Celular, Instituto Butantan, São Paulo, Brazil.
authors.forename: C
authors.initials: C
authors.lastname: Jared
authors.affiliation: Laboratório de Biologia Celular, Instituto Butantan, São Paulo, Brazil.
authors.forename: M M
authors.initials: MM
authors.lastname: Antoniazzi
authors.affiliation: Department of Biomedical Sciences, Cornell University, Ithaca, NY, USA.
authors.forename: E R
authors.initials: ER
authors.lastname: Loew
authors.affiliation: Institute of Ophthalmology, University College London, London, UK.
authors.forename: J K
authors.initials: JK
authors.lastname: Bowmaker
authors.affiliation: Unit of Evolutionary Biology, Zoological Institute, Technical University of Braunschweig, Braunschweig, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Rodriguez
authors.affiliation: School of Animal Biology and The Oceans Institute, The University of Western Australia, Perth, WA, Australia.
authors.forename: N S
authors.initials: NS
authors.lastname: Hart
authors.affiliation: School of Animal Biology and The Oceans Institute, The University of Western Australia, Perth, WA, Australia.|Lions Eye Institute, University of Western Australia, Perth, WA, Australia.
authors.forename: D M
authors.initials: DM
authors.lastname: Hunt
authors.affiliation: School of Animal Biology and The Oceans Institute, The University of Western Australia, Perth, WA, Australia.|School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: J C
authors.initials: JC
authors.lastname: Partridge
authors.affiliation: Department of Life Sciences, The Natural History Museum, London, UK.
authors.forename: D J
authors.initials: DJ
authors.lastname: Gower
mesh_terms: D000818:Animals; D005075:Biological Evolution; D019143:Evolution, Molecular; D008116:Lizards / Q000235:genetics / Q000502:physiology; D008969:Molecular Sequence Data; D055355:Opsins / Q000235:genetics*; D010802:Phylogeny; D012160:Retina / Q000737:chemistry; D012911:Snakes / Q000235:genetics / Q000502:physiology*
chemical_list: D055355:Opsins
keywords: Scolecophidia; Squamata; fossoriality; lizards; opsins; vision