Vertebrate Color Vision Research Articles

Plasticity of cone photoreceptors in adult zebrafish revealed by thyroid hormone exposure

Vertebrate color vision is predominantly mediated by the presence of multiple cone photoreceptor subtypes that are each maximally sensitive to different wavelengths of light. Thyroid hormone (TH) has been shown to be essential in the spatiotemporal patterning of cone subtypes in many species, including cone subtypes that express opsins that are encoded by tandemly replicated genes. TH has been shown to differentially regulate the tandemly replicated lws opsin genes in zebrafish, and exogenous treatments alter the expression levels of these genes in larvae and juveniles. In this study, we sought to determine whether gene expression in cone photoreceptors remains plastic to TH treatment in adults. We used a transgenic lws reporter line, multiplexed fluorescence hybridization chain reaction in situ hybridization, and qPCR to examine the extent to which cone gene expression can be altered by TH in adults. Our studies revealed that opsin gene expression, and the expression of other photoreceptor genes, remains plastic to TH treatment in adult zebrafish. In addition to retinal plasticity, exogenous TH treatment alters skin pigmentation patterns in adult zebrafish after 5 days. Taken together, our results show a remarkable level of TH-sensitive plasticity in the adult zebrafish.

Determining the mechanism of a spectral shift in cone visual pigments using atomistic molecular simulations

Vertebrate color vision requires expression of visual pigments (VPs)with distinct peak spectralsensitivities (λmax)in separate cone photoreceptor populations. VPs consist of a chromophore andassociated opsin protein. λmax is a function of chromophore type and amino acid sequence of the opsin.Minor differences in amino acid sequence can result in large differences in λmax. Elucidating theunderlying mechanisms of this spectral shift in VPs remains a key component to understanding genome tophenome relationships and disease mechanisms, will aid in the development of the field of optogenetics,and will provide a platform for engineering opsin with desirable characteristics.

Ancestral circuits for vertebrate color vision emerge at the first retinal synapse.

For color vision, retinal circuits separate information about intensity and wavelength. In vertebrates that use the full complement of four “ancestral” cone types, the nature and implementation of this computation remain poorly understood. Here, we establish the complete circuit architecture of outer retinal circuits underlying color processing in larval zebrafish. We find that the synaptic outputs of red and green cones efficiently rotate the encoding of natural daylight in a principal components analysis–like manner to yield primary achromatic and spectrally opponent axes, respectively. Blue cones are tuned to capture most remaining variance when opposed to green cones, while UV cone present a UV achromatic axis for prey capture. We note that fruitflies use essentially the same strategy. Therefore, rotating color space into primary achromatic and chromatic axes at the eye’s first synapse may thus be a fundamental principle of color vision when using more than two spectrally well-separated photoreceptor types.

Endocrine regulation of multichromatic color vision

Vertebrate color vision requires spectrally selective opsin-based pigments, expressed in distinct cone photoreceptor populations. In primates and in fish, spectrally divergent opsin genes may reside in head-to-tail tandem arrays. Mechanisms underlying differential expression from such arrays have not been fully elucidated. Regulation of human red (LWS) vs. green (MWS) opsins is considered a stochastic event, whereby upstream enhancers associate randomly with promoters of the proximal or distal gene, and one of these associations becomes permanent. We demonstrate that, distinct from this stochastic model, the endocrine signal thyroid hormone (TH) regulates differential expression of the orthologous zebrafish lws1/lws2 array, and of the tandemly quadruplicated rh2-1/rh2-2/rh2-3/rh2-4 array. TH treatment caused dramatic, dose-dependent increases in abundance of lws1, the proximal member of the lws array, and reduced lws2 Fluorescent lws reporters permitted direct visualization of individual cones switching expression from lws2 to lws1 Athyroidism increased lws2 and reduced lws1, except within a small ventral domain of lws1 that was likely sustained by retinoic acid signaling. Changes in lws abundance and distribution in athyroid zebrafish were rescued by TH, demonstrating plasticity of cone phenotype in response to this signal. TH manipulations also regulated the rh2 array, with athyroidism reducing abundance of distal members. Interestingly, the opsins encoded by the proximal lws gene and distal rh2 genes are sensitive to longer wavelengths than other members of their respective arrays; therefore, endogenous TH acts upon each opsin array to shift overall spectral sensitivity toward longer wavelengths, underlying coordinated changes in visual system function during development and growth.

The Retinal Basis of Vertebrate Color Vision.

The jawless fish that were ancestral to all living vertebrates had four spectral cone types that were probably served by chromatic-opponent retinal circuits. Subsequent evolution of photoreceptor spectral sensitivities is documented for many vertebrate lineages, giving insight into the ecological adaptation of color vision. Beyond the photoreceptors, retinal color processing is best understood in mammals, especially the blueON system, which opposes short- against long-wavelength receptor responses. For other vertebrates that often have three or four types of cone pigment, new findings from zebrafish are extending older work on teleost fish and reptiles to reveal rich color circuitry. Here, horizontal cells establish diverse and complex spectral responses even in photoreceptor outputs. Cone-selective connections to bipolar cells then set up color-opponent synaptic layers in the inner retina, which lead to a large variety of color-opponent channels for transmission to the brain via retinal ganglion cells.

Less is more: lemurs (Eulemur spp.) may benefit from loss of trichromatic vision

Vertebrate color vision is an ideal system for studying the gains and losses of genetic variation across lineages and impacts on behavior. Among placental mammals, trichromatic vision is unique to primates and is argued to be adaptive for foraging on reddish food. However, trichromacy is variably present in lemurs, including species within the cathemeral genus Eulemur, due to inter- and intra-specific variation in X-linked opsin genes. Although this variation could result from genetic drift, it could also reflect ecological adaptation. To understand ecological contributions to color vision variation, we examined cone opsin genes of 11 Eulemur species. We found that only E. flavifrons and E. macaco have polymorphic trichromacy. Most dichromatic species have an “M” (green-shifted) opsin; uniquely, one species (E. rubriventer) has dichromacy based on an “L” (red-shifted) opsin. This latter result appears to represent loss of polymorphic trichromacy from a dichromatic (M opsin) or polymorphic Eulemur ancestor. To address potential ecological explanations for opsin variation, we studied the dietary behavior of wild E. rubriventer and collected reflectance spectra from plant species consumed. Visual models suggest that trichromacy should provide an advantage for detecting reddish foods; however, luminance contrasts were greatest for dichromats with the L opsin. As E. rubriventer are often active in low-light rainforest conditions, luminance cues may be relatively important, which could favor the L opsin, while also leading to relaxed selection on, or selection against, trichromacy. The presence of different opsin alleles across Eulemur species could represent adaptations related to diet, activity pattern, or habitat. Loss of genetic variation, often thought to be maladaptive, can occur through natural selection. Among primates, some species have trichromatic color vision, the ability to distinguish reddish and greenish hues; others are red-green colorblind (dichromatic). We examined adaptive explanations for color vision differences by studying cone opsin genes and behavior in wild lemurs (Eulemur)—a genus that is active both day and night. We found that color vision is variable in Eulemur species, and full trichromatic vision was likely lost in at least one lineage. Foraging ecology of dichromatic Eulemur rubriventer indicates that trichromatic vision should be advantageous for foraging on reddish foods, but brightness cues are more salient to this species’ vision. We suggest brightness may be more important than color to this species, particularly at night, and loss of trichromacy could be adaptive in some lemurs.

Sensory Drive, Color, and Color Vision.

Colors often appear to differ in arbitrary ways among related species. However, a fraction of color diversity may be explained because some signals are more easily perceived in one environment rather than another. Models show that not only signals but also the perception of signals should regularly evolve in response to different environments, whether these primarily involve detection of conspecifics or detection of predators and prey. Thus, a deeper understanding of how perception of color correlates with environmental attributes should help generate more predictive models of color divergence. Here, I briefly review our understanding of color vision in vertebrates. Then I focus on opsin spectral tuning and opsin expression, two traits involved in color perception that have become amenable to study. I ask how opsin tuning is correlated with ecological differences, notably the light environment, and how this potentially affects perception of conspecific colors. Although opsin tuning appears to evolve slowly, opsin expression levels are more evolutionarily labile but have been difficult to connect to color perception. The challenge going forward will be to identify how physiological differences involved in color vision, such as opsin expression levels, translate into perceptual differences, the selection pressures that have driven those differences, and ultimately how this may drive evolution of conspecific colors.

Fine-tuning light sensitivity in the starry flounder (Platichthys stellatus) retina: Regional variation in photoreceptor cell morphology and opsin gene expression.

Vertebrate color vision relies on the differential expression of visual pigment proteins (opsins) in cone photoreceptors of the retina. The diversity of opsins and their retinal expression patterns appear greatest for animals that experience variable light habitats, as is the case with flatfishes. Yet, opsin repertoires and expression patterns in this group of fishes are poorly described. Here, we unveil the visual opsin expression patterns of juvenile starry flounder (Platichthys stellatus) and describe the localization of cone types, their visual pigments and opsin expression. Juvenile starry flounder express eight opsins (Rh1, Sws1, Sws2A1, Sws2A2, Sws2B, Rh2A1, Rh2A2, Lws) and possess a corresponding number of photoreceptor visual pigments, with peak absorbance ranging from 369 to 557 nm. Retinal (vitamin A1) was the only chromophore detected in the retina. Intraretinal variation in opsin abundance consisted of greater expression of both RH2, and lesser expression of SWS1 and both SWS2A, opsin transcripts in the dorsal compared to the ventral retina. Overall cone density was greater in the dorsal retina which was also characterized by a larger proportion of unequal double cones compared with the ventral retina. Together, our results suggest that large opsin repertoires serve to optimize visual function under variable light environments by differential expression of opsin subsets with retinal location.

Vertebrate Bistable Pigment Parapinopsin: Implications for Emergence of Visual Signaling and Neofunctionalization of Non-visual Pigment

Opsins are light-sensor proteins, each absorbing a specific wavelength of light. This, in turn, drives a specific G protein-mediated phototransduction cascade, leading to a photoreceptor cell response. Recent genome projects have revealed an unexpectedly large number of opsin genes for vision and non-visual photoreception in various animals. However, the significance of this multiplicity of opsins remains largely unknown, except for that of cone visual opsins, which are diversified with respect to spectral sensitivity to achieve vertebrate color vision. Here, an implication of multiplicity is discussed, with focus on an ultraviolet-sensitive non-visual opsin—parapinopsin—that underlies pineal wavelength discrimination in lower vertebrates. Parapinopsins are phylogenetically close to vertebrate visual opsins, which have bleaching properties, but interestingly, parapinopsins are bleach-resistant bistable pigments, which photo-convert to stable photoproducts that revert to their original dark state by subsequent light absorption, similar to invertebrate rhodopsins. The unique characteristics of parapinopsin as an evolutionary intermediate between bistable and bleaching pigments provide insight into the evolutionary transition between signaling molecules that interact with two types of opsin-based pigments. Furthermore, the parapinopsin gene was duplicated in the teleost lineage and the spectral sensitivities of the duplicated parapinopsins were different from each other. On the basis of these results, together with the histochemical findings of parapinopsin, a plausible link between the diversification of a non-visual opsin parapinopsin and diverse pineal functions, wavelength discrimination, and melatonin secretion, implying why multiple opsins exist in animals, is proposed.

Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck.

Vertebrate color vision has evolved partly through the modification of five ancestral visual opsin proteins via gene duplication, loss, and shifts in spectral sensitivity. While many vertebrates, particularly mammals, birds, and fishes, have had their visual opsin repertoires studied in great detail, testudines (turtles) and crocodylians have largely been neglected. Here I examine the genomic basis for color vision in four species of turtles and four species of crocodylians, and demonstrate that while turtles appear to vary in their number of visual opsins, crocodylians experienced a reduction in their color discrimination capacity after their divergence from Aves. Based on the opsin sequences present in their genomes and previous measurements of crocodylian cones, I provide evidence that crocodylians have co-opted the rod opsin (RH1) for cone function. This suggests that some crocodylians might have reinvented trichromatic color vision in a novel way, analogous to several primate lineages. The loss of visual opsins in crocodylians paralleled the loss of various anatomical features associated with photoreception, attributed to a "nocturnal bottleneck" similar to that hypothesized for Mesozoic mammals. I further queried crocodylian genomes for nonvisual opsins and genes associated with protection from ultraviolet light, and found evidence for gene inactivation or loss for several of these genes. Two genes, encoding parietopsin and parapinopsin, were additionally inactivated in birds and turtles, likely co-occurring with the loss of the parietal eye in these lineages.

Signals for color and achromatic contrast in the goldfish inner retina

A moving stimulus paradigm was designed to investigate color contrast encoding in the retina. Recently, this paradigm yielded suggestive evidence for color contrast encoding in zebrafish but the significance and generality remain uncertain since the properties of color coding in the zebrafish inner retina are largely unknown. Here, the question of color contrast is pursued in the goldfish retina where there is much accumulated evidence for retinal mechanisms of color vision and opponent color-coding, in particular. Recordings of a sensitive local field potential of the inner retina, the proximal negative response, were made in the intact, superfused retina in the light-adapted state. Responses to color contrast and achromatic contrast were analyzed by comparing responses to a green moving bar on green versus red backgrounds. The quantitative form of the irradiance/response curves was distinctly different under a range of conditions in 32 retinas, thereby providing robust evidence for red-green color contrast. The color contrast is based on successive contrast, occurs in the absence of overt color opponency, and clearly differs from previous findings in the goldfish retina for simultaneous color contrast mediated by color-opponent neurons. The form of the irradiance/response curves suggests that successive color contrast is particularly important when achromatic contrast is low, as often occurs in natural environments. The present results provide a parallel with the well-known principle of human color vision, first proposed by Kirschmann as the third law of color contrast, and may also have implications for the evolution of vertebrate color vision.

Zebrafish inner retina: Local signals for spatial position, luminance, and color contrast

The retina of the zebrafish (Danio rerio) provides an unusually favorable preparation for genetic and developmental studies of the retina. Although the retina has been studied extensively for two decades, the neuronal response of the inner retina is largely unknown. This report describes a prominent local field potential of the inner retina, the Proximal Negative Response (PNR). It is best evoked by small (100 μm) precisely positioned spots of light and is exceedingly sensitive to negative luminance contrast. The polarity, waveform, and other properties of the PNR suggest that it arises primarily from ON-OFF neurons of the proximal retina. The dominant response to negative contrast and its enhancement by light adaptation is believed due to a dominant presynaptic input from OFF bipolar cells. Color contrast was investigated by analyzing responses to a green bar moving on green versus red backgrounds. Over an intermediate range of irradiance, the response to green on red was larger than the response to green on green, thereby providing evidence for the encoding of color contrast. The present findings complement the classic principle of color contrast for human vision known as Kirschmann's third law and bring to mind the view of Walls that color contrast may have been the driving force for the evolution of color vision in lower vertebrates. In sum, the PNR of zebrafish provides clear evidence for the encoding of color and luminance contrast in the inner retina. It exhibits the defining properties common to many other vertebrates, reinforcing the view that the zebrafish may further serve as a model for retinal function and that the PNR may provide a new approach for studies of development, genetics, and retinal degeneration in zebrafish.

Are Avian Retinal Oil Droplets Optically Adapted To Enhance Light Collection In Cone Outer Segments?

Event Abstract Back to Event Are Avian Retinal Oil Droplets Optically Adapted To Enhance Light Collection In Cone Outer Segments? David Wilby1, 2, 3 and Nicholas Roberts1* 1 University of Bristol, School of Biological Sciences, United Kingdom 2 University of Bristol, School of Physics, United Kingdom 3 University of Bristol, Bristol Centre for Functional Nanomaterials, United Kingdom Cone photoreceptors of around half of the vertebrate orders contain spherical optical structures known as oil droplets. These form at the junction between the inner and outer segments. Around half of these oil droplets are also coloured and act as spectral filters, and are believed to improve behavioural colour discrimination. The spherical nature of some of these, in addition to the existence of transparent oil droplets in many vertebrates, implies some optical function beyond that of a filter. The central aim in this work was to determine whether oil droplets are optimised at sub-wavelength scales as a condensing lens to intensify the light field absorbed in cone sub-type. Birds have five types of cone photoreceptor which all have oil droplets, including the primary member of the double cone. In this work, we concentrate on avian cone photoreceptors to study the structure and function of the oil droplet. We present measurements from digital holographic microscopy of chicken retina and electromagnetic models of the spherical oil droplet. Optical phase data and polarisation microscopy suggest a more complex aniostropic structure than a simple isotropic sphere, the presumed structure until now. Digital holographic measurements were used to determine the spatial refractive index properties of the oil droplet. We use these results to inform a sub-wavelgenth scale optical model which has been modelled using electromagnetic frequency- domain finite element analysis. The effect of the oil droplet on the incident light is compared with the morphology of the photoreceptor to determine the evaluate the hypothesis of the oil droplet as a condensing optical element adapted to optimise light capture in each cone sub-type. References G.H. Jacobs, M.P. Rowe, Evolution of vertebrate colour vision, Clin. Exp. Optom., 87 (4-5), 206-216, 2004. N.S. Hart, The Visual Ecology of Avian Photoreceptors, Prog. Retin. Eye Res., 20 (5), 675-703, 2001. Keywords: Colour vision, low-light vision, nanophotonics, optics, photonics, Vision Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster (but consider for participant symposium and student poster award) Topic: Sensory: Vision Citation: Wilby D and Roberts N (2012). Are Avian Retinal Oil Droplets Optically Adapted To Enhance Light Collection In Cone Outer Segments?. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00103 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 24 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Dr. Nicholas Roberts, University of Bristol, School of Biological Sciences, Bristol, BS8 1UG, United Kingdom, nicholas.roberts@bristol.ac.uk Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers David Wilby Nicholas Roberts Google David Wilby Nicholas Roberts Google Scholar David Wilby Nicholas Roberts PubMed David Wilby Nicholas Roberts Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Functional diversity in the color vision of cichlid fishes

BackgroundColor vision plays a critical role in visual behavior. An animal's capacity for color vision rests on the presence of differentially sensitive cone photoreceptors. Spectral sensitivity is a measure of the visual responsiveness of these cones at different light wavelengths. Four classes of cone pigments have been identified in vertebrates, but in teleost fishes, opsin genes have undergone gene duplication events and thus can produce a larger number of spectrally distinct cone pigments. In this study, we examine the question of large-scale variation in color vision with respect to individual, sex and species that may result from differential expression of cone pigments. Cichlid fishes are an excellent model system for examining variation in spectral sensitivity because they have seven distinct cone opsin genes that are differentially expressed.ResultsTo examine the variation in the number of cones that participate in cichlid spectral sensitivity, we used whole organism electrophysiology, opsin gene expression and empirical modeling. Examination of over 100 spectral sensitivity curves from 34 individuals of three species revealed that (1) spectral sensitivity of individual cichlids was based on different subsets of four or five cone pigments, (2) spectral sensitivity was shaped by multiple cone interactions and (3) spectral sensitivity differed between species and correlated with foraging mode and the spectral reflectance of conspecifics. Our data also suggest that there may be significant differences in opsin gene expression between the sexes.ConclusionsOur study describes complex opponent and nonopponent cone interactions that represent the requisite neural processing for color vision. We present the first comprehensive evidence for pentachromatic color vision in vertebrates, which offers the potential for extraordinary spectral discrimination capabilities. We show that opsin gene expression in cichlids, and possibly also spectral sensitivity, may be sex-dependent. We argue that females and males sample their visual environment differently, providing a neural basis for sexually dimorphic visual behaviour. The diversification of spectral sensitivity likely contributes to sensory adaptations that enhance the contrast of transparent prey and the detection of optical signals from conspecifics, suggesting a role for both natural and sexual selection in tuning color vision.

Spectral responses in zebrafish horizontal cells include a tetraphasic response and a novel UV-dominated triphasic response.

Zebrafish are tetrachromats with red (R, 570 nm), green (G, 480 nm), blue (B, 415 nm), and UV (U, 362 nm) cones. Although neurons in other cyprinid retinas are rich in color processing neural circuitry, spectral responses of individual neurons in zebrafish retina, a genetic model for vertebrate color vision, are yet to be studied. Using dye-filled sharp microelectrodes, horizontal cell voltage responses to light stimuli of different wavelengths and irradiances were recorded in a superfused eyecup. Spectral properties were assessed both qualitatively and quantitatively. Six spectral classes of horizontal cell were distinguished. Two monophasic response types (L1 and L2) hyperpolarized at all wavelengths. L1 sensitivities peaked at 493 nm, near the G cone absorbance maximum. Modeled spectra suggest equally weighted inputs from both R and G cones and, in addition, a "hidden opponency" from blue cones. These were classified as R-/G-/(b+). L2 sensitivities were maximal at 563 nm near the R cone absorbance peak; modeled spectra were dominated by R cones, with lesser G cone contributions. B and UV cone signals were small or absent. These are R-/g-. Four chromatic (C-type) horizontal cells were either depolarized (+) or hyperpolarized (-) depending on stimulus wavelength. These types are biphasic (R+/G-/B-) with peak excitation at 467 nm, between G and B cone absorbance peaks, UV triphasic (r-/G+/U-) with peak excitation at 362 nm similar to UV cones, and blue triphasic (r-/G+/B-/u-) and blue tetraphasic (r-/G+/B-/u+), with peak excitation at 409 and 411 nm, respectively, similar to B cones. UV triphasic and blue tetraphasic horizontal cell spectral responses are unique and were not anticipated in previous models of distal color circuitry in cyprinids.

Evolution of colour vision in mammals.

Colour vision allows animals to reliably distinguish differences in the distributions of spectral energies reaching the eye. Although not universal, a capacity for colour vision is sufficiently widespread across the animal kingdom to provide prima facie evidence of its importance as a tool for analysing and interpreting the visual environment. The basic biological mechanisms on which vertebrate colour vision ultimately rests, the cone opsin genes and the photopigments they specify, are highly conserved. Within that constraint, however, the utilization of these basic elements varies in striking ways in that they appear, disappear and emerge in altered form during the course of evolution. These changes, along with other alterations in the visual system, have led to profound variations in the nature and salience of colour vision among the vertebrates. This article concerns the evolution of colour vision among the mammals, viewing that process in the context of relevant biological mechanisms, of variations in mammalian colour vision, and of the utility of colour vision.

Evolution of Dim-Light and Color Vision Pigments

A striking level of diversity of visual systems in different species reflects their adaptive responses to various light environments. To study the adaptive evolution of visual systems, we need to understand how visual pigments, the light-sensitive molecules, have tuned their wavelengths of light absorption. The molecular basis of spectral tuning in visual pigments, a central unsolved problem in phototransduction, can be understood only by studying how different species have adapted to various light environments. Certain amino acid replacements at 30 residues explain some dim-light and color vision in vertebrates. To better understand the molecular and functional adaptations of visual pigments, we must identify all critical amino acid replacements that are involved in the spectral tuning and elucidate the effects of their interactions on the spectral shifts.

Early evolution of multifocal optics for well-focused colour vision in vertebrates

Jawless fishes (Agnatha; lampreys and hagfishes) most closely resemble the earliest stage in vertebrate evolution and lamprey-like animals already existed in the Lower Cambrian [about 540 million years ago (MYA)]. Agnathans are thought to have separated from the main vertebrate lineage at least 500 MYA. Hagfishes have primitive eyes, but the eyes of adult lampreys are well-developed. The southern hemisphere lamprey, Geotria australis, possesses five types of opsin genes, three of which are clearly orthologous to the opsin genes of jawed vertebrates. This suggests that the last common ancestor of all vertebrate lineages possessed a complex colour vision system. In the eyes of many bony fishes and tetrapods, well-focused colour images are created by multifocal crystalline lenses that compensate for longitudinal chromatic aberration. To trace the evolutionary origins of multifocal lenses, we studied the optical properties of the lenses in four species of lamprey (Geotria australis, Mordacia praecox, Lampetra fluviatilis and Petromyzon marinus), with representatives from all three of the extant lamprey families. Multifocal lenses are present in all lampreys studied. This suggests that the ability to create well-focused colour images with multifocal optical systems also evolved very early.

Conserved cis-elements in the Xenopus red opsin promoter necessary for cone-specific expression

The long-wavelength sensitive (red) opsin genes encode proteins which play a central role in daytime and color vision in vertebrates. We used transgenic Xenopus to identify 5′ cis-elements in the red cone opsin promoter necessary for cone-specific expression. We found a highly conserved extended region (−725 to −173) that was required for restricting GFP transgene expression to cones. We further identified a short element (5′-CCAATTAAGAGAT-3′) highly conserved amongst tetrapods, including humans, necessary to restrict expression to cones in the retina. These results identify novel conserved elements that regulate spatial expression of tetrapod red cone opsin genes.

Statistical and Molecular Analyses of Evolutionary Significance of Red-Green Color Vision and Color Blindness in Vertebrates

Red-green color vision is strongly suspected to enhance the survival of its possessors. Despite being red-green color blind, however, many species have successfully competed in nature, which brings into question the evolutionary advantage of achieving red-green color vision. Here, we propose a new method of identifying positive selection at individual amino acid sites with the premise that if positive Darwinian selection has driven the evolution of the protein under consideration, then it should be found mostly at the branches in the phylogenetic tree where its function had changed. The statistical and molecular methods have been applied to 29 visual pigments with the wavelengths of maximal absorption at approximately 510-540 nm (green- or middle wavelength-sensitive [MWS] pigments) and at approximately 560 nm (red- or long wavelength-sensitive [LWS] pigments), which are sampled from a diverse range of vertebrate species. The results show that the MWS pigments are positively selected through amino acid replacements S180A, Y277F, and T285A and that the LWS pigments have been subjected to strong evolutionary conservation. The fact that these positively selected M/LWS pigments are found not only in animals with red-green color vision but also in those with red-green color blindness strongly suggests that both red-green color vision and color blindness have undergone adaptive evolution independently in different species.