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First published online November 28, 2008
Journal of Experimental Biology 211, 3879-3888 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020289

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Ontogenetic changes in photoreceptor opsin gene expression in coho salmon (Oncorhynchus kisutch, Walbaum)

S. E. Temple¹, K. M. Veldhoen¹, J. T. Phelan¹, N. J. Veldhoen² and C. W. Hawryshyn^1,3,*

¹ Department of Biology, University of Victoria, Victoria, British Columbia, Canada
² Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada
³ Department of Biology, Queen's University, Kingston, Ontario, Canada

^* Author for correspondence (e-mail: craig.hawryshyn{at}queensu.ca)

Accepted 13 October 2008

Pacific salmonids start life in fresh water then migrate to the sea, after a metamorphic event called smoltification, later returning to their natal freshwater streams to spawn and die. To accommodate changes in visual environments throughout life history, salmon may adjust their spectral sensitivity. We investigated this possibility by examining ontogenetic and thyroid hormone (TH)-induced changes in visual pigments in coho salmon (Oncorhynchus kisutch, Walbaum). Using microspectrophotometry, we measured the spectral absorbance (quantified by _max) of rods, and middle and long wavelength-sensitive (MWS and LWS) cones in three age classes of coho, representing both freshwater and marine phases. The _max of MWS and LWS cones differed among freshwater (alevin and parr) and ocean (smolt) phases. The _max of rods, on the other hand, did not vary, which is evidence that vitamin A₁/A₂ visual pigment chromophore ratios were similar among freshwater and ocean phases when sampled at the same time of year. Exogenous TH treatment long wavelength shifted the _max of rods, consistent with an increase in A₂. However, shifts in cones were greater than predicted for a change in chromophore ratio. Real-time quantitative RT-PCR demonstrated that at least two RH2 opsin subtypes were expressed in MWS cones, and these were differentially expressed among alevin, parr and TH-treated alevin groups. Combined with changes in A₁/A₂ ratio, differential expression of opsin subtypes allows coho to alter the spectral absorbance of their MWS and LWS cones by as much as 60 and 90 nm, respectively. To our knowledge, this is the largest spectral shift reported in a vertebrate photoreceptor.

Key words: development, retina, fish, teleost, rhodopsin, porphyropsin, isoform, retinal, eye

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