title: Multiple rod-cone and cone-rod photoreceptor transmutations in snakes: evidence from visual opsin gene expression.
abstract: In 1934, Gordon Walls forwarded his radical theory of retinal photoreceptor 'transmutation'. This proposed that rods and cones used for scotopic and photopic vision, respectively, were not fixed but could evolve into each other via a series of morphologically distinguishable intermediates. Walls' prime evidence came from series of diurnal and nocturnal geckos and snakes that appeared to have pure-cone or pure-rod retinas (in forms that Walls believed evolved from ancestors with the reverse complement) or which possessed intermediate photoreceptor cells. Walls was limited in testing his theory because the precise identity of visual pigments present in photoreceptors was then unknown. Subsequent molecular research has hitherto neglected this topic but presents new opportunities. We identify three visual opsin genes, rh1, sws1 and lws, in retinal mRNA of an ecologically and taxonomically diverse sample of snakes central to Walls' theory. We conclude that photoreceptors with superficially rod- or cone-like morphology are not limited to containing scotopic or photopic opsins, respectively. Walls' theory is essentially correct, and more research is needed to identify the patterns, processes and functional implications of transmutation. Future research will help to clarify the fundamental properties and physiology of photoreceptors adapted to function in different light levels.
authors.affiliation: Department of Life Sciences, The Natural History Museum, London SW7 5BD, UK bruno.simoes@me.com.
authors.forename: Bruno F
authors.identifier: http://orcid.org/0000-0001-5390-6541
authors.initials: BF
authors.lastname: Simões
authors.affiliation: Department of Life Sciences, The Natural History Museum, London SW7 5BD, UK.
authors.forename: Filipa L
authors.initials: FL
authors.lastname: Sampaio
authors.affiliation: Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Ellis R
authors.initials: ER
authors.lastname: Loew
authors.affiliation: School of Biological Sciences, University of Adelaide, Adelaide, South Australia 5000, Australia.
authors.forename: Kate L
authors.initials: KL
authors.lastname: Sanders
authors.affiliation: US Geological Survey, Western Ecological Research Center, San Diego, CA 92101, USA.
authors.forename: Robert N
authors.initials: RN
authors.lastname: Fisher
authors.affiliation: Department of Biological Science, Macquarie University, New South Wales 2109, Australia.
authors.forename: Nathan S
authors.initials: NS
authors.lastname: Hart
authors.affiliation: School of Animal Biology, The University of Western Australia, Perth, Western Australia 6009, Australia Lions Eye Institute, University of Western Australia, Perth 6009, Australia.
authors.forename: David M
authors.initials: DM
authors.lastname: Hunt
authors.affiliation: School of Animal Biology, The University of Western Australia, Perth, Western Australia 6009, Australia School of Biological Sciences, University of Bristol, Bristol BS8 1UG, UK.
authors.forename: Julian C
authors.initials: JC
authors.lastname: Partridge
authors.affiliation: Department of Life Sciences, The Natural History Museum, London SW7 5BD, UK d.gower@nhm.ac.uk.
authors.forename: David J
authors.identifier: http://orcid.org/0000-0002-1725-8863
authors.initials: DJ
authors.lastname: Gower
mesh_terms: D000818:Animals; D055358:Cone Opsins / Q000235:genetics / Q000378:metabolism*; D004247:DNA / Q000235:genetics; D005786:Gene Expression Regulation / Q000502:physiology*; D010802:Phylogeny; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012160:Retina / Q000378:metabolism; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology*; D017299:Rod Opsins / Q000235:genetics / Q000378:metabolism*; D012911:Snakes / Q000502:physiology*; D013045:Species Specificity
chemical_list: D055358:Cone Opsins; D012333:RNA, Messenger; D017299:Rod Opsins; D004247:DNA
keywords: Serpentes; mRNA; retina; vision; visual pigments
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