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The Journal of Neuroscience, June 10, 2009, 29(23):7519-7525; doi:10.1523/JNEUROSCI.0517-09.2009

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Cellular/Molecular
Shedding Light on Serpent Sight: The Visual Pigments of Henophidian Snakes

Wayne L. Davies,^1,2 Jill A. Cowing,¹ James K. Bowmaker,¹ Livia S. Carvalho,¹ David J. Gower,³ and David M. Hunt¹

¹UCL Institute of Ophthalmology, London EC1V 9EL, United Kingdom, ²Nuffield Laboratory of Ophthalmology, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom, and ³Department of Zoology, The Natural History Museum, London SW7 5BD, United Kingdom

Correspondence should be addressed to David M. Hunt, UCL Institute of Ophthalmology, 11–43 Bath Street, London EC1V 9EL, UK. Email: d.hunt{at}ucl.ac.uk

The biologist Gordon Walls proposed his "transmutation" theory through the 1930s and the 1940s to explain cone-like morphology of rods (and vice versa) in the duplex retinas of modern-day reptiles, with snakes regarded as the epitome of his hypothesis. Despite Walls' interest, the visual system of reptiles, and in particular snakes, has been widely neglected in favor of studies of fishes and mammals. By analyzing the visual pigments of two henophidian snakes, Xenopeltis unicolor and Python regius, we show that both species express two cone opsins, an ultraviolet-sensitive short-wavelength-sensitive 1 (SWS1) (_max = 361 nm) pigment and a long-wavelength-sensitive (LWS) (_max = 550 nm) pigment, providing the potential for dichromatic color vision. They also possess rod photoreceptors which express the usual rod opsin (Rh1) pigment with a _max at 497 nm. This is the first molecular study of the visual pigments expressed in the photoreceptors of any snake species. The presence of a duplex retina and the characterization of LWS, SWS1, and Rh1 visual pigments in henophidian snakes implies that "lower" snakes do not provide support for Walls' transmutation theory, unlike some "higher" (caenophidian) snakes and other reptiles, such as geckos. More data from other snake lineages will be required to test this hypothesis further.

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Received Jan. 31, 2009; revised April 29, 2009; accepted April 30, 2009.

Correspondence should be addressed to David M. Hunt, UCL Institute of Ophthalmology, 11–43 Bath Street, London EC1V 9EL, UK. Email: d.hunt{at}ucl.ac.uk

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