Spectral Differentiation of Blue Opsins between Phylogenetically Close but Ecologically Distant Goldfish and Zebrafish

Abstract

Zebrafish and goldfish are both diurnal freshwater fish species belonging to the same family, Cyprinidae, but their visual ecological surroundings considerably differ. Zebrafish are surface swimmers in conditions of broad and shortwave-dominated background spectra and goldfish are generalized swimmers whose light environment extends to a depth of elevated short wavelength absorbance with turbidity. The peak absorption spectrum ( max ) of the zebrafish blue (SWS2) visual pigment is consistently shifted to short wavelength (416 nm) compared with that of the goldfish SWS2 (443 nm). Among the amino acid differences between the two pigments, only one (alanine in zebrafish and serine in goldfish at residue 94) was previously known to cause a difference in absorption spectrum (14-nm max shift in newt SWS2). In this study, we reconstructed the ancestral SWS2 pigment of the two species by applying likelihood-based Bayesian statistics and performing sitedirected mutagenesis. The reconstituted ancestral photopigment had a max of 430 nm, indicating that zebrafish and goldfish achieved short wavelength (؊14 nm) and long wavelength (؉13 nm) spectral shifts, respectively, from the ancestor. Unexpectedly, the S94A mutation resulted in only a ؊3-nm spectral shift when introduced into the goldfish SWS2 pigment. Nearly half of the long wavelength shift toward the goldfish pigment was achieved instead by T116L (6 nm). The S295C mutation toward zebrafish SWS2 contributed to creating a ridge of absorbance around 400 nm and broadening its spectral sensitivity in the short wavelength direction. These results indicate that the evolutionary engineering approach is very effective in deciphering the process of functional divergence of visual pigments.Vertebrate visual pigments consist of a protein moiety (i.e. opsin), and a chromophore, either 11-cis-retinal (vitamin A1 aldehyde) or 11-cis 3,4-dehydroretinal (vitamin A2 aldehyde). These pigments reside in rod and cone photoreceptor cells in the retina. Rods are used for dim light vision, and cones are used for daylight and color vision. Vertebrate visual opsins can be classified into five phylogenetic groups: RH1 1 (rod opsin or rhodopsin), RH2 (RH1-like, or green, cone opsin), SWS1 (short wavelength-sensitive type 1, or UV-blue, cone opsin), SWS2 (short wavelength-sensitive type 2, or blue, cone opsin), and M/LWS (middle to long wavelength-sensitive, or red-green, cone opsin) (1). When an opsin forms a photopigment with the A2 chromophore, its peak absorption spectrum ( max ) is located at a longer wavelength than it forms with the A1 chromophore. The spectral shift is wavelength-dependent; it can be as great as 60 nm at longer wavelengths, and as little as 5-10 nm at shorter wavelengths (2, 3).Despite having all five types of opsins in common and sharing ancestry in the same family Cyprinidae, zebrafish (Danio rerio) and goldfish (Carassius auratus) use different chromophores, A1 and A2, respectively, under normal conditions (4 -6). This is consistent with the differences in their visual...

Functional characterization of visual opsin repertoire in Medaka (Oryzias latipes)

“…A Ser164Ala substitution causes a − 7 nm shift in M/LWS opsins (Yokoyama and Radlwimmer, 2001). In addition to the retinal-surrounding sites, a Lue116Thr substitution was reported to cause a − 6 nm shift in goldfish SWS2 (Chinen et al, 2005b) and there is a Ala116Thr difference between medaka SWS2-A and SWS2-B. These aa substitutions may contribute to the spectral difference between the two SWS2 opsins of medaka.…”

“…Among these a Thr118Ala substitution was reported to result in a − 5 nm spectral shift in the SWS2 opsin of cottoid fish (Cowing et al, 2002). A Ser94Ala substitution causes a − 14 nm shift in the newt SWS2 opsin (Takahashi and Ebrey, 2003) but same substitution results only in a − 3 nm shift in goldfish SWS2 (Chinen et al, 2005b). A Ser164Ala substitution causes a − 7 nm shift in M/LWS opsins (Yokoyama and Radlwimmer, 2001).…”

“…Such collective effects could be additive (Chinen et al, 2005b) but are often synergistic (Shi and Yokoyama, 2003) or even regressive (Chinen et al, 2005a), making predictions uncertain of λ max values from only aa variations at key sites. These issues can only be resolved through experimental verification using site-directed mutagenesis.…”

The opsins of the vertebrate retina: insights from structural, biochemical, and evolutionary studies

“…Research with the chameleon (Anolis carolinensis) added sites 49,52,93,207, and 269 to that list, with the spectral shifts shown to occur in an additive fashion [64]. To further show how complicated spectral tuning can be, site 116 was added to the list by observing the absorption difference in goldfish (l max = 443 nm) and zebrafish (l max = 416 nm) SWS2 pigments [65]. Vol.…”

Visual pigments in a palaeognath bird, the emuDromaius novaehollandiae: implications for spectral sensitivity and the origin of ultraviolet vision

“…A number of residues are important for the spectral tuning of SWS2 pigments [19,20]. In particular, a Thr269Ala substitution results in a 10 nm short-wavelength shift [21].…”