Visual system evolution and the nature of the ancestral snake.

The dominant hypothesis for the evolutionary origin of snakes from 'lizards' (non-snake squamates) is that stem snakes acquired many snake features while passing through a profound burrowing (fossorial) phase. To investigate this, we examined the visual pigments and their encoding opsin genes in a range of squamate reptiles, focusing on fossorial lizards and snakes. We sequenced opsin transcripts isolated from retinal cDNA and used microspectrophotometry to measure directly the spectral absorbance of the photoreceptor visual pigments in a subset of samples. In snakes, but not lizards, dedicated fossoriality (as in Scolecophidia and the alethinophidian Anilius scytale) corresponds with loss of all visual opsins other than RH1 (λmax 490-497 nm); all other snakes (including less dedicated burrowers) also have functional sws1 and lws opsin genes. In contrast, the retinas of all lizards sampled, even highly fossorial amphisbaenians with reduced eyes, express functional lws, sws1, sws2 and rh1 genes, and most also express rh2 (i.e. they express all five of the visual opsin genes present in the ancestral vertebrate). Our evidence of visual pigment complements suggests that the visual system of stem snakes was partly reduced, with two (RH2 and SWS2) of the ancestral vertebrate visual pigments being eliminated, but that this did not extend to the extreme additional loss of SWS1 and LWS that subsequently occurred (probably independently) in highly fossorial extant scolecophidians and A. scytale. We therefore consider it unlikely that the ancestral snake was as fossorial as extant scolecophidians, whether or not the latter are para- or monophyletic.

Identification and characterization of visual pigments in caecilians (Amphibia: Gymnophiona), an order of limbless vertebrates with rudimentary eyes.

In comparison with the other amphibian orders, the Anura (frogs) and Urodela (salamanders), knowledge of the visual system of the snake-like Gymnophiona (caecilians) is relatively sparse. Most caecilians are fossorial with, as far as is known any surface activity occurring mainly at night. They have relatively small, poorly developed eyes and might be expected to possess detectable changes in the spectral sensitivity of their visual pigments. Microspectrophotometry was used to determine the spectral sensitivities of the photoreceptors in three species of caecilian, Rhinatrema bivittatum, Geotrypetes seraphini and Typhlonectes natans. Only rod opsin visual pigment, which may be associated with scotopic (dim light) vision when accompanied by other 'rod-specific' components of the phototransduction cascade, was found to be present. Opsin sequences were obtained from the eyes of two species of caecilian, Ichthyophis cf. kohtaoensis and T. natans. These rod opsins were regenerated in vitro with 11-cis retinal to give pigments with spectral sensitivity peaks close to 500 nm. No evidence for cone photoreception, associated with diurnal and colour vision, was detected using molecular and physiological methods. Additionally, visual pigments are short-wavelength shifted in terms of the maximum absorption of light when compared with other amphibian lineages.

The retina of five atherinomorph teleosts: photoreceptors, patterns and spectral sensitivities.

We investigated the spectral and morphological features of the photoreceptors of five atherinomorph teleosts, representing two different orders, and with different life styles and habitats, the Beloniformes and Atheriniformes. The retinae of Belone belone (Belonidae), Dermogenys pusillus (Hemiramphidae), Atherina boyeri (Atherinidae), Marosatherina ladigesi (Telmatherinidae), and Melanotaenia maccullochi (Melanotaeniidae) were examined by light and electron microscopy and microspectrophotometry. In addition to rods, five morphologically different cone types were identified: short, intermediary and long single cones, and double cones which are arranged in distinct specific mosaics. Sporadically, triple cones were also found. Double cones were longer-wave-sensitive, but no general correlation between single cone morphology and spectral sensitivity could be demonstrated. The rods had lambda(max) close to 506-509 nm. The lambda(max) of cone visual pigments ranged from about 368 nm to 578 nm. Ultraviolet-sensitive single cones were present in the three freshwater species, M. ladigesi, M. maccullochi and D. pusillus and three spectrally distinct short-wave-sensitive single cone classes were identified in M. maccullochi. In M. ladigesi, spectral sensitivity varied among individuals due to varying rhodopsin/porphyropsin mixtures. In D. pusillus and M. maccullochi polymorphism of the longer-wave cone pigments might occur. These findings are discussed with respect to phylogeny, photic habitat, behavior and feeding habits.

Vision in the ultraviolet.

Sensitivity to ultraviolet light (UV) is achieved by photoreceptors in the eye that contain a class of visual pigments maximally sensitive to light at wavelengths <400 nm. It is widespread in the animal kingdom where it is used for mate choice, communication and foraging for food. UV sensitivity is not, however, a constant feature of the visual system, and in many vertebrate species, the UV-sensitive (UVS) pigment is replaced by a violet-sensitive (VS) pigment with maximal sensitivity between 410 and 435 nm. The role of protonation of the Schiff base-chromophore linkage and the mechanism for tuning of pigments into the UV is discussed in detail. Amino acid sequence analysis of vertebrate VS/UVS pigments indicates that the ancestral pigment was UVS, with loss of UV sensitivity occurring separately in mammals, amphibia and birds, and subsequently regained by a single amino acid substitution in certain bird species. In contrast, no loss of UV sensitivity has occurred in the UVS pigments of insects.

The molecular basis for spectral tuning of rod visual pigments in deep-sea fish.

Most species of deep-sea fish possess of a rod-only retina with a pigment that is generally shortwave shifted in lambda(max) towards the blue region of the spectrum. In addition, the lambda(max) values of different species tend to cluster at particular points in the spectrum. In this study, the rod opsin gene sequences from 28 deep-sea fish species drawn from seven different Orders are compared. The lambda(max) values of the rod pigments vary from approximately 520 nm to <470 nm, with the majority lying between 490 nm and 477 nm. The 520 nm pigment in two species of dragon fish is associated with a Phe261Tyr substitution, whereas the shortwave shifts of the pigments in the other 26 species are accountable by substitutions at a further eight sites (83, 122, 124, 132, 208, 292, 299 and 300). Clustering of lambda(max) values does not, however, involve a common subset of these substitutions in the different species. A phylogenetic analysis predicts that the pigment in the ancestral species would have had a lambda(max) of approximately 480 nm. A total of 27 changes is required to generate the pattern of substitutions seen in the different species, with many sites undergoing multiple changes.

Rod photopigment deficits in albinos are specific to mammals and arise during retinal development.

Adult albino mammals have specific retinal defects, including reduced numbers of rod photoreceptors. To examine when this rod deficit arises and whether it exists in nonmammalian albinos, we have used absorbance spectrophotometry to measure photopigment levels in dark-adapted eyes taken from three groups of pigmented and albino animals: adult rodents (rats and mice), developing rats, and mature Xenopus frogs. Rhodopsin concentrations were consistently and significantly reduced in mammalian albinos compared to their wild-type counterparts from before the time of eye opening, but photopigment levels were similar in frogs of both pigmentation phenotypes. The results strongly suggest that deficits in the rod cell population arise early in development of the mammalian albino retina, but do not generalize to nonmammalian mutants lacking retinal melanin.

Ionochromic properties of long-wave-sensitive cones in the goldfish retina: an electrophysiological and microspectrophotometric study.

Long-wave-sensitive (LWS) cone visual pigments are sensitive to the concentration of chloride ions and show a spectral shift to shorter wavelengths when exposed to low chloride levels. We have used the aspartate-isolated mass receptor potential of the electroretinogram (ERG) to establish whether the spectrally shifted cone pigment is functional. In the goldfish, Carassius auratus, the lambda(max) of the LWS porphyropsin is displaced from about 622 nm to around 606 nm when chloride is replaced by gluconate. The electrical response of the LWS cones (but not MWS cones and rods) is selectively and reversibly abolished by the removal of chloride ions. The effect is concentration dependent and a decrease to half the normal chloride ion concentration is sufficient to cause a decrease in the response.

Retinal photoreceptors of paleognathous birds: the ostrich (Struthio camelus) and rhea (Rhea americana).

Microspectrophotometry was used to determine the absorbance spectra of both rod and cone visual pigments and oil droplets from the retinae of the ostrich (Struthio camelus) and rhea (Rhea americana). Light and fluorescence microscopy of whole fresh tissue mounts were used to determine the relative numbers and distribution of oil droplets in the retinae. Both species possessed rods, double cones and four classes of single cone identified by their oil droplets. The rods had lambda max at about 505 nm, whereas three cone pigments were recorded with lambda max at 570, 505 and 445 nm. The P570 pigment was located in both members of the double cones and in a class of single cone containing an R-type oil droplet (lambda cut at 555 nm). The P505 and P445 cone pigments were found in populations of single cones containing Y-type and C-type oil droplets (lambda cut of 500 and 420 nm, respectively). The fourth class of single cone contained a T-type droplet and in the ostrich contained a visual pigment with lambda max at about 405 nm. Double cones possessed a P-type droplet in the principal member and an A-type droplet in the accessory member. The complement of visual pigments and oil droplets, and the ratio of cone types in the ostrich and rhea, are remarkably similar to those found in many groups of neognathous birds.

A fully functional rod visual pigment in a blind mammal. A case for adaptive functional reorganization?

In the blind subterranean mole rat Spalax ehrenbergi superspecies complete ablation of the visual image-forming capability has been accompanied by an expansion of the bilateral projection from the retina to the suprachiasmatic nucleus. We have cloned the open reading frame of a visual pigment from Spalax that shows >90% homology with mammalian rod pigments. Baculovirus expression yields a membrane protein with all functional characteristics of a rod visual pigment (lambda(max) = 497 +/- 2 nm; pK(a) of meta I/meta II equilibrium = 6.5; rapid activation of transducin in the light). We not only provide evidence that this Spalax rod pigment is fully functional in vitro but also show that all requirements for a functional pigment are present in vivo. The physiological consequences of this unexpected finding are discussed. One attractive option is that during adaptation to a subterranean lifestyle, the visual system of this mammal has undergone mosaic reorganization, and the visual pigments have adapted to a function in circadian photoreception.

Visual pigment reconstitution in intact goldfish retina using synthetic retinaldehyde isomers.

A protocol has been developed for reconstituting visual pigments in intact retinae by delivering synthetic isomers of retinal incorporated in phospholipid vesicles. Calibration curves have been constructed relating the lambda(max) of the native porphyropsins (visual pigments based on 11-cis 3-dehydroretinal) of the rods and four spectral classes of cone in the goldfish, and the equivalent photosensitive pigments regenerated from 11-cis retinal (rhodopsins) and the commercially available isomer, 9-cis retinal (isorhodopsins). The relationship between the lambda(max) of rhodopsins and isorhodopsins appears to be linear, such that the difference in lambda(max) changes sign at about 380 nm. We therefore conclude that the protocol for reconstituting visual pigments with 9-cis retinal is suitable for all classes of vertebrate opsin-based photopigments.

Spectral tuning of avian violet- and ultraviolet-sensitive visual pigments.

The violet- and ultraviolet-sensitive visual pigments of birds belong to the same class of pigments as the violet-sensitive (so-called blue) pigments of mammals. However, unlike the pigments from mammals and other vertebrate taxa which, depending on species, have lambda(max) values of either around 430 nm or around 370 nm, avian pigments are found with lambda(max) values spread across this range. In this paper, we present the sequences of two pigments isolated from Humbolt penguin and pigeon with intermediate lambda(max) values of 403 and 409 nm, respectively. By comparing the amino acid sequences of these pigments with the true UV pigments of budgerigar and canary and with chicken violet with a lambda(max) value of 420 nm, we have been able to identify five amino acid sites that show a pattern of substitution between species that is consistent with differences in lambda(max). Each of these substitutions has been introduced into budgerigar cDNA and expressed in vitro in COS-7 cells. Only three resulted in spectral shifts in the regenerated pigment; two had relatively small effects and may account for the spectral shifts between penguin, pigeon, and chicken whereas one, the replacement of Ser by Cys at site 90 in the UV pigments, produced a 35 nm shortwave shift that could account for the spectral shift from 403 nm in penguin to around 370 nm in budgerigar and canary.

The ecology of visual pigments.

The visual systems of vertebrates have adapted to function in photic environments ranging from the broad spectrum of full sunlight to almost total darkness, including the restricted spectral ranges found in different coloured aquatic environments. Such adaptations are immediately obvious at the level of retinal photoreceptors. The basic vertebrate photoreceptor pattern consists of rods and four spectrally distinct classes of cone that span the spectrum from the near ultraviolet to the far red. This arrangement is found in many diurnal species including shallow-living teleosts, reptiles and birds, but is noticeably absent in mammals. In freshwater teleosts the visual pigments may be porphyropsins which have maximum sensitivities displaced to longer wavelengths than their equivalent rhodopsins. Water acts as a monochromator, so that with increasing depth the spectral range of the ambient light is restricted, primarily at long wavelengths. Therefore, at depth the down-welling daylight is not only attenuated in intensity, but is restricted to a narrow spectral band centred around 470 nm. Closely related species that live at increasing depths show a loss of long-wave-sensitive cones and a displacement of the maximum sensitivities of middle-wave-sensitive cones and rods to shorter wavelengths. Such species offer a natural model for determining specific amino acids in opsin responsible for the spectral tuning of these middle-wave-sensitive pigments.

Visual pigments and oil droplets in the retina of a passerine bird, the canary Serinus canaria: microspectrophotometry and opsin sequences.

The visual receptors of the passeriform bird Serinus canaria, the canary, have been examined microspectrophotometrically and the sequences of the opsins determined. Rods have a maximum absorbance (lambda max) at 506 nm. Four spectral classes of single cone are present: long-wave-sensitive (LWS) containing a photopigment with lambda max at 569 nm, middle-wave-sensitive (MWS) with lambda max at 505 nm, short-wave-sensitive (SWS) with lambda max at 442 nm, and ultraviolet-sensitive (UVS) with lambda max at about 366 nm. Double cones possess the 569-nm pigment in both members. Typical combinations of photopigment and oil droplet occur in most cone classes. An ambiguity exists in the oil droplet of the single LWS cones. In some birds, LWS cones are paired with an R-type droplet, whereas in the majority of canaries the LWS pigment is paired with a droplet similar to the P-type of double cones. Mechanisms of spectral tuning within each opsin class are discussed.

Morphological changes in the retina of Aequidens pulcher (Cichlidae) after rearing in monochromatic light.

We investigate the processing of chromatic information in the outer retina of a cichlid fish, Aequidens pulcher. The colour opponent response characteristics of some classes of cone-specific horizontal cells in the fish retina are the result of feedforward-feedback loops with cone photoreceptors. To interfere with the reciprocal transmissions of signals, animals were reared in monochromatic lights which preferentially stimulated the spectrally different cone types. Here we report the effects on the cones. Their absorbance spectra were largely unaffected, indicating no change in photopigment gene expression. Significant changes were observed in the cone outer segment lengths and the frequencies of spectral cone types. Quantum catch efficiency and survival of cones appear to be controlled in a spectrally selective way. Our results suggest that the retina responds to spectral deprivation in a compensatory fashion aimed at balancing the input from the different cone types to second order neurons.

What is the function of the cone-rich rim of the retina?

Although there is good histological evidence for a rim of cones extending round the margin of the human retina at the ora serrata, the function of these cones is unknown, and indeed it is not known whether they are functional at all. Four possibilities are discussed here: (i) the cones of the ora serrata may alert us to sudden movements in the far peripheral field, (ii) their signal may be used in estimating optic flow during locomotion, (iii) they may integrate light scattered within the globe of the eye or passing through the sclera, for purposes of colour constancy, or (iv) they may drive circadian rhythms. We report two experiments designed to detect psychophysical correlates of the cone rim. Under the conditions we have used, neither flicker detection nor colour naming show, near the limit of vision, a discontinuity that would correspond to the cone-rich rim.

Evolution of colour vision in vertebrates.

The expression of five major families of visual pigments occurred early in vertebrae evolution, probably about 350-400 million years ago, before the separation of the major vertebrate classes. Phylogenetic analysis of opsin gene sequences suggests that the ancestral pigments were cone pigments, with rod pigments evolving last. Modern teleosts, reptiles and birds have genera that possess rods and four spectral classes of cone each representing one of the five visual pigment families. The complement of four spectrally distinct cone classes endows these species with the potential for tetrachromatic colour vision. In contrast, probably because of their nocturnal ancestry, mammals have rod-dominated retinas with colour vision reduced to a basic dichromatic system subserved by only two spectral classes of cone. It is only within primates, about 35 millions years ago, that mammals 're-evolved' a higher level of colour vision: trichromacy. This was achieved by a gene duplication within the longer-wave cone class to produce two spectrally distinct members of the same visual pigment family which, in conjunction with a short-wavelength pigment, provide the three spectral classes of cone necessary to subserve trichromacy.

The molecular basis for UV vision in birds: spectral characteristics, cDNA sequence and retinal localization of the UV-sensitive visual pigment of the budgerigar (Melopsittacus undulatus).

Microspectrophotometric (msp) studies have shown that the colour-vision system of many bird species is based on four pigments with absorption peaks in the red, green, blue and UV regions of the spectrum. The existence of a fourth pigment (UV) is the major difference between the trichromacy of humans and the tetrachromacy of such birds, and recent studies have shown that it may play a determining role in such diverse aspects of behaviour as mate selection and detection of food. Avian visual pigments are composed of an opsin protein covalently bound via a Schiff-base linkage to the chromophore 11-cis-retinal. Here we report the cDNA sequence of a UV opsin isolated from an avian species, Melopsittacus undulatus (budgerigar or small parakeet). This sequence has been expressed using the recombinant baculovirus system; the pigment generated from the expressed protein on addition of 11-cis-retinal yielded an absorption spectrum typical of a UV photopigment, with lambdamax 365+/-3 nm. This is the first UV opsin from an avian species to be sequenced and expressed in a heterologous system. In situ hybridization of this sequence to budgerigar retinas selectively labelled a sub-set of UV cones, representing approx. 9% of the total cone population, that are distributed in a semi-regular pattern across the entire retina.

Visual Pigments and Molecular Genetics of Color Blindness.

Red/green color blindness, found in ~1 in 15 men, is caused by the expression of hybrid genes coding for visual pigments. Spectral information from site-directed mutagenesis and recombinant expression has led to the possibility of correlating individual genotypes with psychophysical measurements of the severity of the deficiency.

Molecular evolution of trichromacy in primates.

Although trichromacy in Old and New World primates is based on three visual pigments with spectral peaks in the violet (SW, shortwave), green (MW, middlewave) and yellow-green (LW, longwave) regions of the spectrum, the underlying genetic mechanisms differ. The SW pigment is encoded in both cases by an autosomal gene and, in Old World primates, the MW and LW pigments by separate genes on the X chromosome. In contrast, there is a single polymorphic X-linked gene in most New World primates with three alleles coding for spectrally distinct pigments. The one reported exception to this rule is the New World howler monkey that follows the Old World system of separate LW and MW genes. A comparison of gene sequences in these different genetic systems indicates that the duplication that gave rise to the separate MW and LW genes of Old World primates is more ancient than that in the howler monkey. In addition, the amino acid sequences of the two howler monkey pigments show similarities to the pigments encoded by the polymorphic gene of other New World primates. It would appear therefore that the howler monkey gene duplication arose after the split between New and Old World primates and was generated by an unequal crossover that placed two different forms of the New World polymorphic gene on to a single chromosome. In contrast, the lack of identity at variable sites within the New and Old World systems argues for the origin of the separate genes in Old World primates by the duplication of a single form of the gene followed by divergence to give spectrally distinct LW and MW pigments. In contrast, the similarity in amino acid variation across the tri-allelic system of New World primates indicates that this polymorphism had a single origin in New World primates. A striking feature of all these pigments is the use of a common set of substitutions at three amino acid sites to achieve the spectral shift from MW at around 530 nm to LW at around 560 nm. The separate origin of the trichromacy in New and Old World primates would indicate that the selection of these three sites is the result of convergent evolution, perhaps as a consequence of visual adaptation in both cases to foraging for yellow and orange fruits against a green foliage.