The rhodopsin-porphyropsin system in freshwater fishes. 1. Effects of age and photic environment

Vitamin A1/A2 chromophore exchange: Its role in spectral tuning and visual plasticity

Vertebrate rod and cone photoreceptors detect light via a specialized organelle called the outer segment. This structure is packed with light-sensitive molecules known as visual pigments that consist of a G-protein-coupled, seven-transmembrane protein known as opsin, and a chromophore prosthetic group, either 11-cis retinal ('A₁') or 11-cis 3,4-didehydroretinal ('A₂'). The enzyme cyp27c1 converts A₁ into A₂ in the retinal pigment epithelium. Replacing A₁ with A₂ in a visual pigment red-shifts its spectral sensitivity and broadens its bandwidth of absorption at the expense of decreased photosensitivity and increased thermal noise. The use of vitamin A₂-based visual pigments is strongly associated with the occupation of aquatic habitats in which the ambient light is red-shifted. By modulating the A₁/A₂ ratio in the retina, an organism can dynamically tune the spectral sensitivity of the visual system to better match the predominant wavelengths of light in its environment. As many as a quarter of all vertebrate species utilize A₂, at least during a part of their life cycle or under certain environmental conditions. A₂ utilization therefore represents an important and widespread mechanism of sensory plasticity. This review provides an up-to-date account of the A₁/A₂ chromophore exchange system.

Visual adaptation in Lake Victoria cichlid fishes: depth-related variation of color and scotopic opsins in species from sand/mud bottoms

Background

For Lake Victoria cichlid species inhabiting rocky substrates with differing light regimes, it has been proposed that adaptation of the long-wavelength-sensitive (LWS) opsin gene triggered speciation by sensory drive through color signal divergence. The extensive and continuous sand/mud substrates are also species-rich, and a correlation between male nuptial coloration and the absorption of LWS pigments has been reported. However, the factors driving genetic and functional diversity of LWS pigments in sand/mud habitats are still unresolved.

Results

To address this issue, nucleotide sequences of eight opsin genes were compared in ten Lake Victoria cichlid species collected from sand/mud bottoms. Among eight opsins, the LWS and rod-opsin (RH1) alleles were diversified and one particular allele was dominant or fixed in each species. Natural selection has acted on and fixed LWS alleles in each species. The functions of LWS and RH1 alleles were measured by absorption of reconstituted A1- and A2-derived visual pigments. The absorption of pigments from RH1 alleles most common in deep water were largely shifted toward red, whereas those of LWS alleles were largely shifted toward blue in both A1 and A2 pigments. In both RH1 and LWS pigments, A2-derived pigments were closer to the dominant light in deep water, suggesting the possibility of the adaptation of A2-derived pigments to depth-dependent light regimes.

Conclusions

The RH1 and LWS sequences may be diversified for adaptation of A2-derived pigments to different light environments in sand/mud substrates. Diversification of the LWS alleles may have originally taken place in riverine environments, with a new mutation occurring subsequently in Lake Victoria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-1040-x) contains supplementary material, which is available to authorized users.

Presence of rhodopsin and porphyropsin in the eyes of 164 fishes, representing marine, diadromous, coastal and freshwater species--a qualitative and comparative study

There are two types of visual pigments in fish eyes; most marine fishes have rhodopsin, while most freshwater fishes have porphyropsin. The biochemical basis for this dichotomy is the nature of the chromophores, retinal (A1) and 3-dehydroretinal (A2), each of which is bound by an opsin. In order to study the regional distribution of these visual pigments, we performed a new survey of the visual pigment chromophores in the eyes of many species of fish. Fish eyes from 164 species were used to examine their chromophores by high-performance liquid chromatography--44 species of freshwater fish, 20 of peripheral freshwater fish (coastal species), 10 of diadromous fish and 90 of seawater fish (marine species) were studied. The eyes of freshwater fish, limb freshwater fish and diadromous fish had both A1 and A2 chromophores, whereas those of marine fish possessed only A1 chromophores. Our results are similar to those of previous studies; however, we made a new finding that fish which live in freshwater possessed A1 if living near the sea and A2 if living far from the sea if they possessed only one type of chromophore.

SENSORY TRADE-OFFS PREDICT SIGNAL DIVERGENCE IN SURFPERCH

Unidirectional elaboration of male trait evolution (e.g., larger, brighter males) has been predicted by receiver bias models of sexual selection and empirically tested in a number of different taxa. This study identifies a bidirectional pattern of male trait evolution and suggests that a sensory constraint is driving this divergence. In this system, the inherent trade-off in dichromatic visual detection places limits on the direction that sensory biases may take and thus provides a quantitative test of the sensory drive model. Here I show that sensory systems with trade-offs in detection abilities produce bidirectional biases and that signal design properties match these biases. I combine species-specific measurements and ancestral estimates with visual detection modeling to examine biases in sensory and signaling traits across five fish species occupying optically diverse habitats in the Californian kelp forest. Species-specific divergence in visual pigments correlates with changes in environment and produces different sensory biases--favoring luminance (brightness) detection for some species and chromatic (color) detection for others. Divergence in male signals (spectral reflectance of orange, blue, and silver color elements) is predicted by each species' sensory bias: color divergence favors chromatic detection for species with chromatically biased visual systems, whereas species with luminance sensory biases have signals favoring luminance detection. This quantitative example of coevolution of communication traits varying in a bidirectional pattern governed by the environment is the first demonstration of sensory trade-offs driving signal evolution.

Seasonal variation of chromophore composition in the eye of the Japanese dace, Tribolodon hakonensis

The relationship between seasonal variation and the effect of several different environmental factors on chromophore composition was investigated in the eye of the Japanese dace, Tribolodon hakonensis which lives either in rivers or in the sea. Eyes obtained from river and sea populations had both retinal (A1) and 3,4-didehydroretinal (A2) all through the year but the ratio of these chromophores showed seasonal variation the relative amount of A2 was higher in winter and lower in summer. Besides seasonal variation, A2 showed marked differences depending on habitat: the highest proportion of A2 was 67% in January and the lowest 13% in July, in the river population, whereas in the sea population the highest and the lowest values were only 30 and 6%, respectively, during the same months. The seasonal variation in gonadosomatic index showed no correlation to variations in A2 proportion, and the maximum difference in water temperature between summer and winter was ca. 15 degrees C for both habitats. Because spectral conditions at the locations of capture of both river and sea populations were similar, we conclude that Japanese dace eyes are affected by exogenous factors related to differences between freshwater and seawater environments.

The thermal contribution to photoactivation in A2 visual pigments studied by temperature effects on spectral properties

Effects of temperature on the spectral properties of visual pigments were measured in the physiological range (5-28 degrees C) in photoreceptor cells of bullfrog (Rana catesbeiana) and crucian carp (Carassius carassius). Absorbance spectra recorded by microspectrophotometry (MSP) in single cells and sensitivity spectra recorded by electroretinography (ERG) across the isolated retina were combined to yield accurate composite spectra from ca. 400 nm to 800 nm. The four photoreceptor types selected for study allowed three comparisons illuminating the properties of pigments using the dehydroretinal (A2) chromophore: (1) the two members of an A1/A2 pigment pair with the same opsin (porphyropsin vs. rhodopsin in bullfrog "red" rods); (2) two A2 pigments with similar spectra (porphyropsin rods of bullfrog and crucian carp); and (3) two A2 pigments with different spectra (rods vs. long-wavelength-sensitive (L-) cones of crucian carp). Qualitatively, the temperature effects on A2 pigments were similar to those described previously for the A1 pigment of toad "red" rods. Warming caused an increase in relative sensitivities at very long wavelengths but additionally a small shift of lambdamax toward shorter wavelengths. The former effect was used for estimating the minimum energy required for photoactivation (Ea) of the pigment. Bullfrog rod opsin with A2 chromophore had Ea = 44.2 +/- 0.9 kcal/mol, significantly lower (one-tailed P < 0.05) than the value Ea = 46.5 +/- 0.8 kcal/mol for the same opsin coupled to A1. The A2 rod pigment of crucian carp had Ea = 42.3 +/- 0.6 kcal/mol, which is significantly higher (one-tailed P < 0.01) than that of the L-cones in the same retina (Ea = 38.3 +/- 0.4 kcal/mol), whereas the difference compared with the bullfrog A2 rod pigment is not statistically significant (two-tailed P = 0.13). No strict connection between lambdamax and Ea appears to exist among A2 pigments any more than among A1 pigments. Still, the A1 --> A2 chromophore substitution in bullfrog opsin causes three changes correlated as originally hypothesized by Barlow (1957): a red-shift of lambdamax, a decrease in Ea, and an increase in thermal noise.

Morphology and spectral sensitivities of retinal and extraretinal photoreceptors in freshwater teleosts

Fish eyes possess a complicated morphological and neural organisation of retinal and extra-retinal receptors. Features such as photoreceptor mosaic array and photoreceptor grouping are unique among vertebrates. Spectral sensitivities of these photoreceptors range from UV to the red portion of the visible spectrum. Moreover, these sensitivities can change with the age of the animals. In this review we will examine thoroughly the morphology, and the spectral sensitivities of retinal and extra-retinal receptors and the influence upon them of factors such as hormones, ageing, season, habitat light conditions, and migration.

Visual pigments and the photic environment: the cottoid fish of Lake Baikal

The endemic cottoid fish of Lake Baikal in Eastern Siberia offer a singular opportunity for examining within a number of closely related species, the relationships of visual pigments, photoreceptor complements and depth within a deep freshwater environment. The lake, the deepest (1600 m) and one of the largest and most ancient in the world, is unique in that the oxygen levels at the bottom are only reduced to about 80% of the surface levels. We have studied, by light microscopy, microspectrophotometry and visual pigment extraction, the retinas from 17 species of Baikal cottoids that live at different depths within the lake. Generally the retinas contain, in addition to rods, large green-sensitive double cones and small blue-sensitive single cones: surprisingly for freshwater fish, the visual pigments are based on Vitamin A1. The lambda max of both rods and cones are displaced to shorter wavelengths with increasing depth. Surface species have cones with lambda max at about 546, 525 and 450 nm and rods at 523 nm, deeper living species retain cones, but with lambda max shifting towards 500 and 425 nm and with rods at 480 nm, whereas the deepest living fish possess only rods (lambda max 480-500 nm). These data clearly show a correlation between photoreceptor complement, visual pigment lambda max and depth, but question the hypothesis that there is a correlation of pigment lambda max with water colour since, in contrast to oceanic waters, the maximum transmission of Baikal water is between 550 and 600 nm.

Expression of rod and cone visual pigments in goldfish and zebrafish: a rhodopsin-like gene is expressed in cones

The primary purpose of the present study was to determine whether a rhodopsin-like gene, which has been postulated to represent the green cone pigment in several species, is in fact expressed in cone photoreceptors instead of rods. The expression patterns of rod opsin and blue and red cone opsins were also examined in both goldfish and zebrafish retinas using colorimetric in situ hybridization. The results demonstrate that the rhodopsin-like gene is expressed in green cones, as predicted. A subset of small cones that do not hybridize with these cRNA probes are tentatively identified as ultraviolet receptors. The results also demonstrate that opsin message in cones is restricted to the perinuclear region, whereas in rods, it is both perinuclear and adjacent to the ellipsoid.

A visual pigment of the sturgeon retina

The visual pigments of hybrid sturgeon (a cross between Acipenser ruthenus (male) and Huso huso (female) have been studied both by the methods of incomplete partial bleaching and HPLC analysis. On the basis of the results obtained the relationship between the structure of opsins and the spectral characteristics of visual pigments is discussed.

Competition between retinol and 3,4-didehydroretinol for esterification in crude pigment epithelial cell fractions

The membrane fraction of the retinal pigment epithelium (RPE) of the frog (Rana pipiens) catalyzed the esterification of tritiated retinol to retinyl esters. This esterification reaction was inhibited in the presence of 3,4-didehydroretinol.

Visual pigments and the labile scotopic visual system of fish

Among mammals, birds, most reptiles and chondrichthians, only rhodopsins are present. Among agnathans, osteichthians, amphibians and certain freshwater turtles there are species having only porphyropsins or only rhodopsins or, more interestingly, both pigments, either sequentially or together. This latter grouping represents the paired-pigment species. Associated with the presence of paired-pigments is the possibility that the proportions of rhodopsin and porphyropsin may change. Depending on the characteristics of each paired-pigment species, naturally occurring changes in visual pigment ratios are related to migrations in anadromous and catadromous teleosts and anadromous cyclostomes and to seasonal variation in several teleosts. In addition, the visual pigment composition of certain species of teleosts has been altered by the specific effects of light, temperature, diet and hormones. Of two possible mechanisms for altering spectral sensitivity, varying the proportion of rhodopsin and porphyropsin is far more common than utilizing a single chromophore and changing the opsin. In addition to the long established evidence that extractable rod pigment ratios may change during the life cycle or in response to specific exogenous factors, there is the more recent recognition from microspectrophotometry that cone pigment ratios may also change in concert. The effect of lighting conditions and temperature on the visual pigment composition of certain paired-pigment species is presented.

Seasonal variation of the vitamin A2-based visual pigment in the retina of adult bullfrog, Rana catesbeiana

Visual pigment composition was determined by HPLC analysis over a one-year period in the adult bullfrog (Rana catesbeiana). In Japan, the vitamin A2-based pigment was only 5% of the total visual pigment from the middle of July to October. The vitamin A2-based pigment increased in November and reached a maximum of 32-36% between January and June. This seasonal variation may relate to the average of outdoor temperature rather than the daylight hours. The amount of vitamin A2-based pigment began to increase when the average temperature became lower than 20 degrees C and it decreased rapidly as the average temperature was higher than 20 degrees C.

Visual pigment changes in rainbow trout in response to temperature

Lower water temperature (6 degrees C in comparison to 16 degrees C) favors a higher proportion of porphyropsin in the retina of rainbow trout (Salmo gairdneri), regardless of the light conditions (constant darkness or 12 hours of light and 12 of darkness). This response to temperature does not follow a Q10 relation, namely an increase in the rate of a reaction produced by raising the temperature 10 degrees C.

Rhodopsin and porphyropsin fields in the adult bullfrog retina

Though it had been supposed earlier that the bullfrog undergoes a virtually complete metamorphosis of visual systems from vitamin A(2) and porphyropsin in the tadpole to vitamin A(1) and rhodopsin in the adult, the present observations show that the retina of the adult frog may contain as much as 30-40% porphyropsin, all of it segregated in the dorsal zone. The most dorsal quarter of the adult retina may contain 81-89% porphyropsin mixed with a minor amount of rhodopsin; the ventral half contains only rhodopsin. Further, the dorsal zone contains a two to three times higher concentration of visual pigments than the ventral retina. The pigment epithelium underlying the retina contains a corresponding distribution of vitamins A(1) and A(2), predominantly vitamin A(2) in the dorsal pigment epithelium, exclusively vitamin A(1) in the ventral zone. The retina accepts whatever vitamin A the pigment epithelium provides it with, and turns it into the corresponding visual pigment. Thus, a piece of light-adapted dorsal retina laid back on ventral pigment epithelium regenerates rhodopsin, whereas a piece of light-adapted ventral retina laid back on dorsal pigment epithelium regenerates predominantly porphyropsin. Vitamin A(2) must be made from vitamin A(1), by dehydrogenation at the 3,4-bond in the ring. This conversion must occur in the pigment epithelium, presumably through the action of a vitamin A-3,4-dehydrogenase. The essential change at metamorphosis is to make much less of this dehydrogenase, and to sequester it in the dorsal pigment epithelium. Some adult bullfrogs, perhaps characteristically taken in the summer, contain very little porphyropsin-only perhaps 5%-still sequestered in the dorsal retina. The gradient of light over the retinal surface has little if any effect on this distribution. The greater density of visual pigments in the dorsal retina, and perhaps also-although this is less clear-the presence of porphyropsin in this zone, has some ecological importance in increasing the retinal sensitivity to the dimmer and, on occasion, redder light received from below.