Understanding why organisms possess particular combinations of visual pigments (VPs) is central to visual ecology. Species that adjust spectral sensitivity by changing retinal VPs provide a powerful tool to investigate this question. Changing chromophores (A1 or A2) and altering opsin expression are mechanisms used to adjust photoreceptor maximum absorbance (λmax). My research explored the function of such dynamic systems in two teleosts, coho salmon and zebrafish. I investigated temporal VP changes in relation to life history and environmental change. In coho, I found a correlation between chromophore shifts and seasonal variations in environmental variables for freshwater and marine life history stages (seasonal hypothesis). These findings provide an alternative explanation to the migration/metamorphosis hypothesis. The latter, suggests that shifts from A-2 to AI at metamorphosis, preceding seaward migration, are preemptive to changes in photic environment. Using exogenous thyroid hormone (TH), which plays a role in coho metamorphosis, I demonstrated that under various rearing conditions and times of year, TH consistently shifted VPs towards A2 dominance. This increased A2 is opposite to that occurring at metamorphosis, further supporting the seasonal hypothesis. While TH induced changes in rod λmax were consistent with a change in A1/A2 ratio, λmax variations in middle wavelength- and long wavelength-sensitive (MWS and LWS) cones were greater than predicted by a shift in A1/A2 ratio alone. I proposed a change in opsin expression to explain MWS and LWS cone λmax variations. In support of this hypothesis, a novel RH2 opsin subtype (expressed in MWS cones) was isolated and sequenced. This second RH2 possessed an E to Q substitution at the position analogous to 122 in bovine RH1 which imposes a hypsochromic shift in X. Further investigation found that changes in coho MWS cone λmax were correlated with ontogeny and the frequency of MWS cones with below 500 nm was reduced in marine compared to freshwater stages. The combination of changes in A1/A2 ratio and opsin expression provides coho with a dynamic spectral tuning mechanism. In zebrafish, I demonstrated the presence of an A1/A2 VP pair, which shifted to A2 dominance with exogenous TH treatment, but not with temperature.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/2295 |
| Date | 03 March 2010 |
| Creators | Temple, Shelby Eric |
| Contributors | Hawryshyn, Craig W. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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