Abstract
Glacial-relict species of the genus Mysis (opossum shrimps) inhabiting both fresh-water lakes and brackish sea waters in northern Europe show a consistent lake/sea dichotomy in eye spectral sensitivity. The absorbance peak (λmax) recorded by microspectrophotometry in isolated rhabdoms is invariably 20–30 nm red-shifted in “lake” compared with “sea” populations. The dichotomy holds across species, major opsin lineages and light environments. Chromophore exchange from A1 to A2 (retinal → 3,4-didehydroretinal) is a well-known mechanism for red-shifting visual pigments depending on environmental conditions or stages of life history, present not only in fishes and amphibians, but in some crustaceans as well. We tested the hypothesis that the lake/sea dichotomy in Mysis is due to the use of different chromophores, focussing on two populations of M. relicta from, respectively, a Finnish lake and the Baltic Sea. They are genetically very similar, having been separated for less than 10 kyr, and their rhabdoms show a typical lake/sea difference in λmax (554 nm vs. 529 nm). Gene sequencing has revealed no differences translating into amino acid substitutions in the transmembrane parts of their opsins. We determined the chromophore identity (A1 or A2) in the eyes of these two populations by HPLC, using as standards pure chromophores A1 and A2 as well as extracts from bovine (A1) and goldfish (A2) retinas. We found that the visual-pigment chromophore in both populations is A1 exclusively. Thus the spectral difference between these two populations of M. relicta is not due to the use of different chromophores. We argue that this conclusion is likely to hold for all populations of M. relicta as well as its European sibling species.
Highlights
The crustacean genus Mysis encompasses a large number of species and populations living in marine, brackish-water and freshwater environments
Inter- and intraspecific spectral differences could not be consistently explained by amino acid substitutions in the opsins, whether in residues implicated in spectral tuning of vertebrate pigments [9,10,11,12] or any other loci [7]
The addition of the A1+A2 standard mixture to the extract is associated with a new peak in the A2 position, not present in the bovine extract alone. These results are as such completely unsurprising. They serve to confirm that our all-trans standards and corresponding natural chromophore from retinal extracts behave identically under our HPLC protocol
Summary
The crustacean genus Mysis encompasses a large number of species and populations living in marine, brackish-water and freshwater environments. Repeated changes of habitats and light environments have occurred at both inter- and intraspecific levels on time scales ranging from millions to a few thousands of years [1]. This makes Mysis an attractive clade for studying rates and modes of visual evolution in relation to photic environment. There is a consistent spectral dichotomy between ‘‘lake’’ and ‘‘sea’’ populations, such that spectral absorbance in all lake populations studied (n = 5) peaks at significantly longer wavelengths (range of within-population mean lmax = 554–562 nm) than in any of the sea populations (lmax = 521–535 nm; n = 4). Inter- and intraspecific spectral differences could not be consistently explained by amino acid substitutions in the opsins, whether in residues implicated in spectral tuning of vertebrate pigments [9,10,11,12] or any other loci [7]
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