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J. Pahlberg, A. Audzijonyte, R. Vainola, K. Donner; Molecular Evolution of the Opsin Gene in the Opossum Shrimp Mysis: Adaptation to Photic Conditions vs. Phylogenetic Constraints. Invest. Ophthalmol. Vis. Sci. 2007;48(13):611.
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© ARVO (1962-2015); The Authors (2016-present)
Using the circumboreal Mysis relicta species group (Crustacea, Mysida) as a model, we elucidate to what extent and on what time scale molecular evolution of opsin is driven by adaptation to different light environments and how often analogous changes in different species occur.
Opossum shrimps of the Mysis relicta species group comprise four "glacial relict" sibling species, distributed in boreal lakes, the Baltic Sea, and circumarctic coastal waters. Two of these species were studied here, M. relicta and M. salemaai. The absorbance spectrum of the visual pigment of populations from different, spectrally characterized light environments in the Baltic Sea and Finnish and Swedish lakes was measured in situ in single rhabdoms by microspectrophotometry (MSP). The wavelength of maximum absorbance (max) and chromophore identity of the visual pigment was determined for each individual separately by fitting Govardovskii et al. (2000) templates to the across-rhabdom average spectrum for that individual. DNA was isolated from identified individuals of different populations and used for genomic DNA sequencing, and total RNA was extracted, isolated and used for cDNA synthesis.
Only one visual pigment was found in any of the populations examined and all used only A2 chromophore. The most striking result is the complete segregation of max between "Lake" (max typically ca. 555 nm) and "Sea" populations (max typically ca. 530 nm) in both species (always different at least at P < 0.01). Still, the present spectral transmittance of the water in several lakes is not in fact very different from that of the Baltic Sea. Most puzzlingly, however, sequencing of the opsin gene has so far revealed no amino acid differences that could explain the differences in max of the M. relicta populations.
In view of the postglacial development of the Baltic Sea region, the "Lake"/"Sea" shift in max is generally consistent with adaptive tuning of the opsin for improved quantum catch on a time scale of ca.10.000 years. Possible reasons and implications of our failure to find amino acid substitutions underlying the max differences of M.relicta will be discussed.
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