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First published online June 13, 2008
Journal of Experimental Biology 211, 2134-2143 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009365

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Effects of exogenous thyroid hormones on visual pigment composition in coho salmon (Oncorhynchus kisutch)

Shelby E. Temple¹, Samuel D. Ramsden¹, Theodore J. Haimberger¹, Kathy M. Veldhoen¹, Nik J. Veldhoen², Nicolette L. Carter¹, Wolff-Michael Roth³ and Craig W. Hawryshyn^1,4,*

¹ Department of Biology, University of Victoria, Victoria, British Columbia, Canada
² Department of Microbiology and Biochemistry, University of Victoria, Victoria, British Columbia, Canada
³ Faculty of Education Research, University of Victoria, Victoria, British Columbia, Canada
⁴ Department of Biology and Center for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada

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

Accepted 23 April 2008

The role of exogenous thyroid hormone on visual pigment content of rod and cone photoreceptors was investigated in coho salmon (Oncorhynchus kisutch). Coho vary the ratio of vitamin A₁- and A₂-based visual pigments in their eyes. This variability potentially alters spectral sensitivity and thermal stability of the visual pigments. We tested whether the direction of shift in the vitamin A₁/A₂ ratio, resulting from application of exogenous thyroid hormone, varied in fish of different ages and held under different environmental conditions. Changes in the vitamin A₁/A₂ visual pigment ratio were estimated by measuring the change in maximum absorbance (_max) of rods using microspectrophotometry (MSP). Exogenous thyroid hormone resulted in a long-wavelength shift in rod, middle-wavelength-sensitive (MWS) and long-wavelength-sensitive (LWS) cone photoreceptors. Rod and LWS cone _max values increased, consistent with an increase in vitamin A₂. MWS cone _max values increased more than predicted for a change in the vitamin A₁/A₂ ratio. To account for this shift, we tested for the expression of multiple RH2 opsin subtypes. We isolated and sequenced a novel RH2 opsin subtype, which had 48 amino acid differences from the previously sequenced coho RH2 opsin. A substitution of glutamate for glutamine at position 122 could partially account for the greater than predicted shift in MWS cone _max values. Our findings fit the hypothesis that a variable vitamin A₁/A₂ ratio provides seasonality in spectral tuning and/or improved thermal stability of visual pigments in the face of seasonal environmental changes, and that multiple RH2 opsin subtypes can provide flexibility in spectral tuning associated with migration–metamorphic events.

Key words: rhodopsin, porphyropsin, thyroxine, fish, vision, opsin gene sequence, expression, PCR, MSP

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