Rods are rods and cones cones, and (never) the twain shall meet.

More than 100 photopigment G protein-coupled receptors (opsins) have been sequenced and organized into six classes. Rod photoreceptors in various species have been found to express an opsin from one of the two rhodopsin classes, while cones express an opsin from one of the four remaining classes. It has now been discovered that salamander short-wavelength sensitive cones and green rods express the same opsin, while manifesting other features that classically distinguish rods from cones.     

Population variation in opsin expression in the bluefin killifish,<Emphasis Type="Italic"> Lucania goodei</Emphasis>: a real-time PCR study

Quantitative genetics have not been used in vision studies because of the difficulty of objectively measuring large numbers of individuals. Here, we examine the effectiveness of a molecular technique, real-time PCR, as an inference of visual components in the bluefin killifish, Lucania goodei, to determine whether there is population variation in opsin expression. Previous work has shown that spring animals possess a higher frequency of UV and violet cones and a lower frequency of yellow and red cones than swamp animals. Here, we found a good qualitative match between the population differences in opsin expression and those found previously in cone frequency. Spring animals expressed higher amounts of SWS1 and SWS2B opsins (which correspond to UV and violet photopigments) and lower amounts of RH2 and LWS opsins (which correspond to yellow and red photopigments) than swamp animals. The counterintuitive pattern between color pattern, lighting environment, and vision remains. Males with blue anal fins are more abundant in swamps where animals express fewer SWS1 and SWS2B opsins and where transmission of UV/blue wavelengths is low. Understanding this system requires quantitative genetic studies. Real-time PCR is an effective tool for studies requiring inferences of visual physiology in large numbers of individuals.Abbreviations ERG electroretinogram - MSP microspectrophotometry     

The Action of 11-cis-Retinol on Cone Opsins and Intact Cone Photoreceptors

11-cis-Retinol has previously been shown in physiological experiments to promote dark adaptation and recovery of photoresponsiveness of bleached salamander red cones but not of bleached salamander red rods. The purpose of this study was to evaluate the direct interaction of 11-cis-retinol with expressed human and salamander cone opsins, and to determine by microspectrophotometry pigment formation in isolated salamander photoreceptors. We show here in a cell-free system using incorporation of radioactive guanosine 5′-3-O-(thio)triphosphate into transducin as an index of activity, that 11-cis-retinol inactivates expressed salamander cone opsins, acting an inverse agonist. Similar results were obtained with expressed human red and green opsins. 11-cis-Retinol had no significant effect on the activity of human blue cone opsin. In contrast, 11-cis-retinol activates the expressed salamander and human red rod opsins, acting as an agonist. Using microspectrophotometry of salamander cone photoreceptors before and after bleaching and following subsequent treatment with 11-cis-retinol, we show that 11-cis-retinol promotes pigment formation. Pigment was not formed in salamander red rods or green rods (containing the same opsin as blue cones) treated under the same conditions. These results demonstrate that 11-cis-retinol is not a useful substrate for rod photoreceptors although it is for cone photoreceptors. These data support the premise that rods and cones have mechanisms for handling retinoids and regenerating visual pigment that are specific to photoreceptor type. These mechanisms are critical to providing regenerated pigments in a time scale required for the function of these two types of photoreceptors.11-cis-Retinol is the precursor to 11-cis-retinal, the 11-cis-aldehyde form of vitamin A and the chromophore that combines covalently with rod and cone opsin proteins to form visual pigments. 11-cis-Retinal is consumed during visual signaling, and its continual synthesis is required. Photon absorption by the visual pigments causes the isomerization of its chromophore to the all-trans configuration. This initiates two processes critical for vision: activation of the photoreceptor cell and the eventual recovery of the original photosensitivity of the cells, requiring regeneration of the visual pigments. As cones are used for bright light vision, these two processes must work more rapidly in cones than in rods and thus cones have a higher requirement of 11-cis-retinoids as suggested by Rushton (1, 2).Photoreceptor activation begins with photoisomerization of the chromophore within the visual pigment. This results in a subsequent conformational change of the protein part of the visual pigment that is able to activate its G protein transducin, which in turn activates a PDE that lowers the concentration of cGMP and closes cGMP-gated ion channels. These steps comprise the visual signal transduction cascade (see Ref. 3 for review).The visual cycle involves regeneration of the visual pigment, which ultimately deactivates the protein and accomplishes the recovery of the photosensitivity of the photoreceptor cell. Classically, this process involves both the photoreceptor cell and the retinal pigment epithelium (RPE).⁴ After photoisomerization of the chromophore and formation of the active visual pigment, all-trans-retinal is released from the opsin and reduced to all-trans-retinol, which is then transported to the RPE where it is isomerized to 11-cis-retinol through a number of steps. In the RPE, 11-cis-retinol is oxidized to the aldehyde form, which is transported back to the photoreceptor cell and can be directly used by all of the opsins to regenerate an inactive pigment ready for photoactivation. The details of this model have been extensively reviewed (4, 5). Alternatively, recent work suggests that cones have an additional source of 11-cis-retinoids from Müller cells (6–8). Like the RPE cells, Müller cells have been shown to be able to convert all-trans-retinol to 11-cis-retinol (6). Unlike in the RPE cells, 11-cis-retinol is not oxidized to 11-cis-retinal in Müller cells.Jones et al. (9) demonstrated that administration of 11-cis-retinol to bleached salamander red cones could restore photosensitivity. A logical conclusion was that red cones were able to oxidize 11-cis-retinol to the aldehyde and regenerate visual pigments although noncovalent binding of 11-cis-retinol to red cone opsins generating a light-sensitive complex could not be excluded. On the other hand, 11-cis-retinol does not restore photosensitivity to bleached salamander rod cells but appears to directly activate the cells (9, 10). The data suggested that the rods were not able to oxidize 11-cis-retinol, but that the retinol itself could activate the signal transduction cascade, and indeed we recently demonstrated that 11-cis-retinol acts as an agonist to expressed bovine rod opsin (11). Our aim here was to study the action of 11-cis-retinol on cone opsins and cone photoreceptor cells to determine the efficacy of an alternate visual cycle for cones.The photoreceptor cells used in this study are from tiger salamander, and the expressed opsins used for biochemical experiments are those from salamander and human. Photoreceptor cells are generally identified by cell morphology and the type of opsin it contains that can be further complicated by the findings that some cone cells have multiple opsins (12, 13). Recently genetic analysis has determined that opsins fall into five classes (reviewed in Refs. 14 and 15). We have studied opsins falling into four of these classes and use common color-derived names for the opsins and photoreceptor cells. The classic rod cells used for scotopic vision contain rhodopsin, the visual pigment for the rod opsin (RH1 opsin) and appeared red and thus have been designated as red rods. Some species such as salamanders have an additional rod cell whose photosensitivity is blue-shifted from that of the red rod and thus designated as green rods. In the tiger salamander, the green rods contain the identical opsin (SWS2 opsin) found in blue cones (16). The human blue cones contain an opsin from a different class (SWS1 opsin), which is homologous to the salamander UV cone opsin. The human red and green and salamander red cone opsins all belong to the same class of opsins (M/LWS opsins). Absorption properties of visual pigments are further modulated in some animals including the tiger salamander by use of 11-cis-retinal with an additional double bond (3,4-dehydro or A2 11-cis-retinal) resulting in red-shifted absorbance from pigments containing 11-cis-retinal (A1 11-cis-retinal).We show here that 11-cis-retinol is not an agonist to cone opsins and does not itself generate a light-sensitive opsin. We further show using microspectrophotometry that both red and blue salamander cone cells regenerate visual pigments from 11-cis-retinol, whereas pigments could not be regenerated with 11-cis-retinol in bleached salamander red and green rods even though the latter contains the same opsin as the salamander blue cone. Thus, rods and cones have mechanisms for handling retinoids and regenerating visual pigment that are specific to photoreceptor type, and these mechanisms are critical to providing regenerated pigments in a time scale required for the function of these two types of photoreceptors.     

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

A variety of visual pigment repertoires present in fish species is believed due to the great variation under the water of light environment. A complete set of visual opsin genes has been isolated and characterized for absorption spectra and expression in the retina only in zebrafish. Medaka (Oryzias latipes) is a fish species phylogenetically distant from zebrafish and has served as an important vertebrate model system in molecular and developmental genetics. We previously isolated a medaka rod opsin gene (RH1). In the present study we isolated all the cone opsin genes of medaka by genome screening of a lambda-phage and bacterial artificial chromosome (BAC) libraries. The medaka genome contains two red, LWS-A and LWS-B, three green, RH2-A, RH2-B and RH2-C, and two blue, SWS2-A and SWS2-B, subtype opsin genes as well as a single-copy of the ultraviolet, SWS1, opsin gene. Previously only one gene was believed present for each opsin type as reported in a cDNA-based study. These subtype opsin genes are closely linked and must be the products of local gene duplications but not of a genome-wide duplication. Peak absorption spectra (lambda(max)) of the reconstituted photopigments with 11-cis retinal varied greatly among the three green opsins, 452 nm for RH2-A, 516 nm for RH2-B and 492 nm for RH2-C, and between the two blue opsins, 439 nm for SWS2-A and 405 nm for SWS2-B. Zebrafish also has multiple opsin subtypes, but phylogenetic analysis revealed that medaka and zebrafish gained the subtype opsins independently. The lambda and BAC DNA clones isolated in this study could be useful for investigating the regulatory mechanisms and evolutionary diversity of fish opsin genes.     

Characterization of photoreceptor cell types in the little brown bat Myotis lucifugus (Vespertilionidae)

We report the expression of three visual opsins in the retina of the little brown bat (Myotis lucifugus, Vespertilionidae). Gene sequences for a rod-specific opsin and two cone-specific opsins were cloned from cDNA derived from bat eyes. Comparative sequence analyses indicate that the two cone opsins correspond to an ultraviolet short-wavelength opsin (SWS1) and a long-wavelength opsin (LWS). Immunocytochemistry using antisera to visual opsins revealed that the little brown bat retina contains two types of cone photoreceptors within a rod-dominated background. However, unlike other mammalian photoreceptors, M. lucifugus cones and rods are morphologically indistinguishable by light microscopy. Both photoreceptor types have a thin, elongated outer segment. Using microspectrophotometry we classified the absorption spectrum for the ubiquitous rods. Similar to other mammals, bat rhodopsin has an absorption peak near 500 nm. Although we were unable to confirm a spectral range, cellular and molecular analyses indicate that M. lucifugus expresses two types of cone visual pigments located within the photoreceptor layer. This study provides important insights into the visual capacity of a nocturnal microchiropteran species.     

Molecular cloning of cone opsin genes and their expression in the retina of a smelt, Ayu (Plecoglossus altivelis, Teleostei)

Five cone opsin genes of landlocked ayu fish (Plecoglossus altivelis) were cloned, and the expression patterns of these genes were investigated. AYU-LWS, -RH2-1, -RH2-2, -SWS1-1, and -SWS1-2 were isolated and had high (more than 75%) identity with red, green, green, UV, and UV-sensitive opsin, respectively, genes of other fish reported previously. The results of Southern blotting experiments showed that each gene is present as a single copy. Gene expression was measured by RT-PCR using four populations collected from rivers and a lake in spring and summer. The results of the RT-PCR experiment showed that AYU-SWS1-2 was highly expressed, whereas AYU-SWS1-1 was scarce. Two RH2 opsins were expressed simultaneously in the same individual, and the expression ratio between these opsins changed among populations. In situ hybridization revealed that AYU-LWS and -RH2-1 were expressed in the double cones and that AYU-RH2-2 and -SWS1-2 were expressed in the long and short single cones (LSC and SSC), respectively. It was shown that an individual ayu expresses two RH2 opsins simultaneously in different types of cone cells.     

Molecular Evidence that Only Two Opsin Subfamilies,the Blue Light- (SWS2) and Green Light-Sensitive (RH2), Drive Color Vision in Atlantic Cod (Gadus morhua)

Teleosts show a great variety in visual opsin complement, due to both gene duplication and gene loss. The repertoire ranges from one subfamily of visual opsins (scotopic vision) including rod opsin only retinas seen in many deep-sea species to multiple subfamilies of visual opsins in some pelagic species. We have investigated the opsin repertoire of Atlantic cod (Gadus morhua) using information in the recently sequenced cod genome and found that despite cod not being a deep sea species it lacks visual subfamilies sensitive towards the most extreme parts of the light spectra representing UV and red light. Furthermore, we find that Atlantic cod has duplicated paralogs of both blue-sensitive SWS2 and green-sensitive RH2 subfamilies, with members belonging to each subfamily linked in tandem within the genome (two SWS2-, and three RH2A genes, respectively). The presence of multiple cone opsin genes indicates that there have been duplication events in the cod ancestor SWS2 and RH2 opsins producing paralogs that have been retained in Atlantic. Our results are supported by expressional analysis of cone opsins, which further revealed an ontogenetic change in the array of cone opsins expressed. These findings suggest life stage specific programs for opsin regulation which could be linked to habitat changes and available light as the larvae is transformed into an early juvenile. Altogether we provide the first molecular evidence for color vision driven by only two families of cone opsins due to gene loss in a teleost.     

A visual pigment expressed in both rod and cone photoreceptors.

Rods and cones contain closely related but distinct G protein-coupled receptors, opsins, which have diverged to meet the differing requirements of night and day vision. Here, we provide evidence for an exception to that rule. Results from immunohistochemistry, spectrophotometry, and single-cell RT-PCR demonstrate that, in the tiger salamander, the green rods and blue-sensitive cones contain the same opsin. In contrast, the two cells express distinct G protein transducin alpha subunits: rod alpha transducin in green rods and cone alpha transducin in blue-sensitive cones. The different transducins do not appear to markedly affect photon sensitivity or response kinetics in the green rod and blue-sensitive cone. This suggests that neither the cell topology or the transducin is sufficient to differentiate the rod and the cone response.     

Rapid light-induced shifts in opsin expression: finding new opsins, discerning mechanisms of change, and implications for visual sensitivity

Light-induced shifts in cone frequency and opsin expression occur in many aquatic species. Yet little is known about how quickly animals can alter opsin expression and, thereby, track their visual environments. Similarly, little is known about whether adult animals can alter opsin expression or whether shifts in opsin expression are limited to critical developmental windows. We took adult wild-caught bluefin killifish (Lucania goodei) from three different lighting environments (spring, swamp and variable), placed them under two different lighting treatments (clear vs. tea-stained water) and monitored opsin expression over 4 weeks. We measured opsin expression for five previously described opsins (SWS1, SWS2B, SWS2A, RH2-1 and LWS) as well as RH2-2 which we discovered via 454 sequencing. We used two different metrics of opsin expression. We measured expression of each opsin relative to a housekeeping gene and the proportional expression of each opsin relative to the total pool of opsins. Population and lighting environment had large effects on opsin expression which were present at the earliest time points indicating rapid shifts in expression. The two measures of expression produced radically different patterns. Proportional measures indicated large effects of light on SWS1 expression, whereas relative measures indicated no such effect. Instead, light had large effects on the relative expression of SWS2B, RH2-2, RH2-1 and LWS. We suggest that proportional measures of opsin expression are best for making inferences about colour vision, but that measures relative to a housekeeping gene are better for making conclusions about which opsins are differentially regulated.     

Identification of cis-acting elements repressing blue opsin expression in zebrafish UV cones and pineal cells

Opsin genes are expressed in a cell type-specific manner in the retina and the pineal organ for visual and nonvisual photoreceptive purposes, but the regulatory mechanism behind the tissue and cell selectivity is not well understood. In this study, we focus on the expression regulation of the blue-sensitive opsin gene SWS2 of zebrafish by taking a transgenic approach using the green fluorescence protein as an expression reporter. The zebrafish SWS2 is a single-copy gene and is expressed specifically in the "long single cones" in the retina. We found the following. 1) A 0.3-kb region between 0.6 and 0.3 kb 5' of the SWS2 initiation codon, encompassing four cone-rod homeobox-binding sites (OTX sequences), contains the region necessary and sufficient to drive gene expression in long single cones. 2) A 15-bp portion (-341 to -327) in the 0.3-kb region represses the gene expression in the "short single cones," which are dedicated to the UV-sensitive opsin gene SWS1. 3) An 11-bp sequence TAACTGCCAGT (-441 to -431) in the 0.3-kb region, with its adjacent OTX element, also works as a repressor for gene expression in the pineal cells. 4) Finally, this OTX site is necessary for expression repression in the bipolar cells in the retina. These findings open a way for understanding the complex interaction of positive and negative regulatory factors that govern the cell type specificity of the opsin gene expression in the photoreceptive cells in the retina and the pineal organ. We termed the novel 11-bp sequence as the pineal negative regulatory element, PINE.     

Photoreceptors and visual pigments in three species of newts

Photoreceptor composition and retinal visual pigments in three newt (Caudata, Salamandridae, Pleurodelinae) species (Pleurodeles waltl, Lissotriton (Triturus) vulgaris, and Cynops orientalis) were studied by light microscopy and single-cell microspectrophotometry. Retinas of all three species contain “red” (rhodopsin/porphyropsin) rods, large and small single cones, and double cones. Large single cones and both components of double cones contain red-sensitive (presumably LWS) visual pigment whose absorption spectrum peaks between 593 and 611 nm. Small single cones are either blue- (SWS2, maximum absorption between 470 and 489 nm) or UV-sensitive (SWS1, maximum absorption between 340 and 359 nm). Chromophore composition of visual pigments (A₁ vs. A₂) was assessed both from template fitting of absorption spectra and by the method of selective bleaching. All pigments contained a mixture of A₁ (11-cis retinal) and A₂ (11-cis-3,4-dehydroretinal) chromophore in the proportion depending on the species and cell type. In all cases, A₂ was dominant. However, in C. orientalis rods the fraction of A₁ could reach 45%, while in P. waltl and L. vulgaris cones it did not exceed 5%. Remarkably, the absorption of the newt blue-sensitive visual pigment was shifted by up to 45 nm toward the longer wavelength, as compared with all other amphibian SWS2-pigments. We found no “green” rods typical of retinas of Anura and some Caudata (ambystomas) in the three newt species studied.     

Ancestral Loss of Short Wave-Sensitive Cone Visual Pigment in Lorisiform Prosimians,Contrasting with Its Strict Conservation in Other Prosimians

Mammals are basically dichromatic in color vision, possessing middle to long wave-sensitive (M/LWS) and the short wave-sensitive (SWS) cone opsins in the retina, whereas some nocturnal mammals lack functional SWS opsins. Prosimians, primitive primates consisting of three extant groups (Lorisiformes, Lemuriformes, and Tarsiiformes), include many nocturnal species. Among nocturnal prosimians, a species of lorisiforms, the greater galago (Otolemur crassicaudatus), is known to lack a functional SWS opsin gene, while lemuriforms and tarsiiforms appear to retain SWS opsins in the retina. It has not been established, however, whether the loss of SWS opsin is a universal phenomenon among lorisiforms and whether the functional SWS opsin genes of lemuriforms and tarsiiforms are under strict or relaxed selective constraint. To gain better insight into an association between nocturnality and loss of SWS function, we isolated and sequenced the SWS opsin genes from two species of lorisiforms, the slow loris (Nycticebus coucang; nocturnal) and the lesser galago (Galago senegalensis; nocturnal), and one species each of lemuriforms and tarsiiforms, the brown lemur (Eulemur fulvus; cathemeral) and the western tarsier (Tarsius bancanus; nocturnal), respectively. Our sequence analysis revealed that (1) the SWS opsin gene was disrupted in the common ancestor of galagids and lorisids and (2) the rate of nonsynonymous nucleotide substitution has been kept significantly lower than that of synonymous substitution in tarsier and lemur, demonstrating the presence of strict selective constraint on the SWS opsin genes in tarsiiforms and lemuriforms.     

Vertebrate opsins belonging to different classes vary in constitutively active properties resulting from salt-bridge mutations

Vertebrate opsins are classified into one of five classes on the basis of amino acid similarity. These classes are short wavelength sensitive 1 and 2 (SWS1, SWS2), medium/long wavelength sensitive (M/LWS), and rod opsin like 1 and 2 (RH1, RH2). In bovine rod opsin (RH1), two critical amino acids form a salt bridge in the apoprotein that maintains the opsin in an inactive state. These residues are K296, which functions as the chromophore binding site, and E113, which functions as the counterion to the protonated Schiff base. Corresponding residues in each of the other vertebrate opsin classes are believed to play similar roles. Previous reports have demonstrated that mutations in these critical residues result in constitutive activation of transducin by RH1 class opsins in the absence of chromophore. Additionally, recent reports have shown that an E113Q mutation in SWS1 opsin is constitutively active. Here we ask if the other classes of vertebrate opsins maintain activation characteristics similar to that of bovine RH1 opsin. We approach this question by making the corresponding substitutions which disrupt the K296/E113 salt bridge in opsins belonging to the other vertebrate opsin classes. The mutant opsins are tested for their ability to constitutively activate bovine transducin. We demonstrate that mutations disrupting this key salt bridge produce constitutive activation in all classes. However, the mutant opsins differ in their ability to be quenched in the dark state by the addition of chromophore as well as in their level of constitutive activation. The differences in constitutive activation profiles suggest that structural differences exist among the opsin classes that may translate into a difference in activation properties.     

Gene duplication and spectral diversification of cone visual pigments of zebrafish

Zebrafish is becoming a powerful animal model for the study of vision but the genomic organization and variation of its visual opsins have not been fully characterized. We show here that zebrafish has two red (LWS-1 and LWS-2), four green (RH2-1, RH2-2, RH2-3, and RH2-4), and single blue (SWS2) and ultraviolet (SWS1) opsin genes in the genome, among which LWS-2, RH2-2, and RH2-3 are novel. SWS2, LWS-1, and LWS-2 are located in tandem and RH2-1, RH2-2, RH2-3, and RH2-4 form another tandem gene cluster. The peak absorption spectra (lambdamax) of the reconstituted photopigments from the opsin cDNAs differed markedly among them: 558 nm (LWS-1), 548 nm (LWS-2), 467 nm (RH2-1), 476 nm (RH2-2), 488 nm (RH2-3), 505 nm (RH2-4), 355 nm (SWS1), 416 nm (SWS2), and 501 nm (RH1, rod opsin). The quantitative RT-PCR revealed a considerable difference among the opsin genes in the expression level in the retina. The expression of the two red opsin genes and of three green opsin genes, RH2-1, RH2-3, and RH2-4, is significantly lower than that of RH2-2, SWS1, and SWS2. These findings must contribute to our comprehensive understanding of visual capabilities of zebrafish and the evolution of the fish visual system and should become a basis of further studies on expression and developmental regulation of the opsin genes.     

VA opsin,melanopsin, and an inherent light response within retinal interneurons

BACKGROUND: Although photoreception is best understood in rods and cones, it is increasingly clear that these are not the only photoreceptive cells of the vertebrate retina. While considerable attention has been paid to the role of melanopsin in the generation of intrinsic light sensitivity in the retinal ganglion cells of mammals, nothing is known about the photoreceptive capacity of the horizontal cells of the fish retina in which both VA opsin and melanopsin are expressed. As yet, there has been little more than speculation as to the physiological function of these opsins within local retinal circuit neurons. RESULTS: VA opsin and melanopsin have been isolated and localized within the well-characterized cyprinid retina of the roach (Rutilus rutilus). Parallel electrophysiological studies identified a novel subtype of horizontal cell (HC-RSD) characterized by a depolarizing response that fits an opsin photopigment with a lambda(max) of 477 nm. The HC-RSD cells mediate responses to light that are characterized by long integration times, well beyond those observed for rods and cones. Significantly, HC-RSD responses persist when the conventional photoreceptor inputs are saturated by background light. CONCLUSIONS: The syncytium of coupled horizontal cells has long been considered to provide a signal of overall retinal irradiance. Our data suggest that this light information is, at least in part, derived from a population of intrinsically photosensitive VA opsin and/or melanopsin horizontal cells.     

Bat Eyes Have Ultraviolet-Sensitive Cone Photoreceptors

Mammalian retinae have rod photoreceptors for night vision and cone photoreceptors for daylight and colour vision. For colour discrimination, most mammals possess two cone populations with two visual pigments (opsins) that have absorption maxima at short wavelengths (blue or ultraviolet light) and long wavelengths (green or red light). Microchiropteran bats, which use echolocation to navigate and forage in complete darkness, have long been considered to have pure rod retinae. Here we use opsin immunohistochemistry to show that two phyllostomid microbats, Glossophaga soricina and Carollia perspicillata, possess a significant population of cones and express two cone opsins, a shortwave-sensitive (S) opsin and a longwave-sensitive (L) opsin. A substantial population of cones expresses S opsin exclusively, whereas the other cones mostly coexpress L and S opsin. S opsin gene analysis suggests ultraviolet (UV, wavelengths <400 nm) sensitivity, and corneal electroretinogram recordings reveal an elevated sensitivity to UV light which is mediated by an S cone visual pigment. Therefore bats have retained the ancestral UV tuning of the S cone pigment. We conclude that bats have the prerequisite for daylight vision, dichromatic colour vision, and UV vision. For bats, the UV-sensitive cones may be advantageous for visual orientation at twilight, predator avoidance, and detection of UV-reflecting flowers for those that feed on nectar.     

Why UV vision and red vision are important for damselfish (Pomacentridae): structural and expression variation in opsin genes

Coral reefs belong to the most diverse ecosystems on our planet. The diversity in coloration and lifestyles of coral reef fishes makes them a particularly promising system to study the role of visual communication and adaptation. Here, we investigated the evolution of visual pigment genes (opsins) in damselfish (Pomacentridae) and examined whether structural and expression variation of opsins can be linked to ecology. Using DNA sequence data of a phylogenetically representative set of 31 damselfish species, we show that all but one visual opsin are evolving under positive selection. In addition, selection on opsin tuning sites, including cases of divergent, parallel, convergent and reversed evolution, has been strong throughout the radiation of damselfish, emphasizing the importance of visual tuning for this group. The highest functional variation in opsin protein sequences was observed in the short‐ followed by the long‐wavelength end of the visual spectrum. Comparative gene expression analyses of a subset of the same species revealed that with SWS1, RH2B and RH2A always being expressed, damselfish use an overall short‐wavelength shifted expression profile. Interestingly, not only did all species express SWS1 – a UV‐sensitive opsin – and possess UV‐transmitting lenses, most species also feature UV‐reflective body parts. This suggests that damsels might benefit from a close‐range UV‐based ‘private’ communication channel, which is likely to be hidden from ‘UV‐blind’ predators. Finally, we found that LWS expression is highly correlated to feeding strategy in damsels with herbivorous feeders having an increased LWS expression, possibly enhancing the detection of benthic algae.