Evolution of vertebrate visual pigments

How Oratosquilla oratoria compound eye response to the polarization of light: In the perspective of vision genes and related proteins

The special rhabdom structure of the mid-band ommatidium in compound eye contributes to the mantis shrimp being the only animal species known to science that can recognize circularly polarized light (CPL). Although the number of mid-band ommatidium of Oratosquilla oratoria is reduced, the mid-band ommatidium still has orthogonal geometric interleaved rhabdom and short oval distal rhabdom, which may mean that the O. oratoria has weakened circular polarized light vision (CPLV). Here we explored the molecular mechanisms of how O. oratoria response to the polarization of light. Based on the specific expression patterns of vision-related functional genes and proteins, we suggest that the order of light response by O. oratoria compound eye was first natural light, then left-circularly polarized light (LCPL), linearly polarized light, right-circularly polarized light (RCPL) and dark. Meanwhile, we found that the expression levels of vision-related functional genes and proteins in O. oratoria compound eye under RCPL were not significantly different from those in DL, which may imply that O. oratoria cannot respond to RCPL. Furthermore, the response of LCPL is likely facilitated by the differential expression of opsin and microvilli - related functional genes and proteins (arrestin and sodium-coupled neutral amino acid transporter). In conclusion, this study systematically illustrated for the first time how O. oratoria compound eye response to the polarization of light at the genetic level, and it can improve the visual ecological theory behind polarized light vision evolution.

Evolution of visual guanylyl cyclases and their activating proteins with respect to clade and species-specific visual system adaptation

Membrane guanylyl cyclase receptors are important regulators of local cGMP production, critically influencing cell growth and differentiation as well as ion transport, blood pressure and calcium feedback of vertebrate phototransduction. Currently, seven different subtypes of membrane guanylyl cyclase receptors have been characterized. These receptors have tissue specific expression and are activated either by small extracellular ligands, changing CO2 concentrations or, in the case of visual guanylyl cyclases, intracellularly interacting Ca2+-dependent activating proteins. In this report, we focus on the visual guanylyl cyclase receptors (GCs) GC-E (gucy2d/e) and GC-F (gucy2f) and their activating proteins (GCAP1/2/3; guca1a/b/c). While gucy2d/e has been detected in all analyzed vertebrates, GC-F receptors are missing in several clades (reptiles, birds, and marsupials) and/or individual species. Interestingly, the absence of GC-F in highly visual sauropsida species with up to 4 different cone-opsins is compensated by an increased number of guanylyl cyclase activating proteins, whereas in nocturnal or visually impaired species with reduced spectral sensitivity it is consolidated by the parallel inactivation of these activators. In mammals, the presence of GC-E and GC-F is accompanied by the expression of one to three GCAPs, whereas in lizards and birds, up to five different GCAPs are regulating the activity of the single GC-E visual membrane receptor. In several nearly blind species, a single GC-E enzyme is often accompanied by a single variant of GCAP, suggesting that one cyclase and one activating protein are both sufficient and required for conferring the basic detection of light.

Thyroid Hormone Signaling Is Required for Dynamic Variation in Opsins in the Retina during Metamorphosis of the Japanese Flounder (Paralichthys olivaceus)

In the present study, we investigated the function of thyroid hormones (TH) in visual remodeling during Japanese flounder (Paralichthys olivaceus) metamorphosis through cellular molecular biology experiments. Our results showed that the expression of the five opsin genes of the flounder were highest in eye tissue and varied with the metamorphosis process. The expression of rh1, sws2aβ and lws was positively regulated by exogenous TH, but inhibited by thiourea (TU) compared to the control group. In addition, there was a significant increase in sws2aβ and lws in the rescue experiments performed with TU-treated larvae (p < 0.05). Meanwhile, T3 levels in flounder larvae were increased by TH and decreased by TU. Based on the differences in the expression of the three isoforms of the thyroid hormone receptor (TR) (Trαa, Trαb and Trβ), we further hypothesized that T3 may directly or indirectly regulate the expression of sws2aβ through Trαa. This study demonstrates the regulatory role of TH in opsins during flounder metamorphosis and provides a basis for further investigation on the molecular mechanisms underlying the development of the retinal photoreceptor system in flounders.

Insect visual sensitivity to long wavelengths enhances colour contrast of insects against vegetation

The sensitivity of animal photoreceptors to different wavelengths of light strongly influence the perceived visual contrast of objects in the environment. Outside of the human visual wavelength range, ultraviolet sensitivity in many species provides important and behaviourally relevant visual contrast between objects. However, at the opposite end of the spectrum, the potential advantage of red sensitivity remains unclear. We investigated the potential benefit of long wavelength sensitivity by modelling the visual contrast of a wide range of jewel beetle colours against flowers and leaves of their host plants to hypothetical insect visual systems. We find that the presence of a long wavelength sensitive photoreceptor increases estimated colour contrast, particularly of beetles against leaves. Moreover, under our model parameters, a trichromatic visual system with ultraviolet (λ_max = 355 nm), short (λ_max = 445 nm) and long (λ_max = 600 nm) wavelength photoreceptors performed as well as a tetrachromatic visual system, which had an additional medium wavelength photoreceptor (λ_max = 530 nm). When we varied λ_max for the long wavelength sensitive receptor in a tetrachromatic system, contrast values between beetles, flowers and leaves were all enhanced with increasing λ_max from 580 nm to at least 640 nm. These results suggest a potential advantage of red sensitivity in visual discrimination of insect colours against vegetation and highlight the potential adaptive value of long wavelength sensitivity in insects.

Simultaneous expression of UV and violet SWS1 opsins expands the visual palette in a group of freshwater snakes

Snakes are known to express a rod visual opsin and two cone opsins, only (SWS1, LWS), a reduced palette resulting from their supposedly fossorial origins. Dipsadid snakes in the genus Helicops are highly visual predators that successfully invaded freshwater habitats from ancestral terrestrial-only habitats. Here, we report the first case of multiple SWS1 visual pigments in a vertebrate, simultaneously expressed in different photoreceptors and conferring both UV and violet sensitivity to Helicops snakes. Molecular analysis and in vitro expression confirmed the presence of two functional SWS1 opsins, likely the result of recent gene duplication. Evolutionary analyses indicate that each sws1 variant has undergone different evolutionary paths with strong purifying selection acting on the UV-sensitive copy and d_N/d_S ∼1 on the violet-sensitive copy. Site-directed mutagenesis points to the functional role of a single amino acid substitution, Phe86Val, in the large spectral shift between UV and violet opsins. In addition, higher densities of photoreceptors and SWS1 cones in the ventral retina suggest improved acuity in the upper visual field possibly correlated with visually guided behaviors. The expanded visual opsin repertoire and specialized retinal architecture are likely to improve photon uptake in underwater and terrestrial environments, and provide the neural substrate for a gain in chromatic discrimination, potentially conferring unique color vision in the UV–violet range. Our findings highlight the innovative solutions undertaken by a highly specialized lineage to tackle the challenges imposed by the invasion of novel photic environments and the extraordinary diversity of evolutionary trajectories taken by visual opsin-based perception in vertebrates.

Long-Wavelength-Filtered Light Transiently Inhibits Negative Lens-Induced Axial Eye Growth in the Chick Myopia Model

Purpose

Eye growth and myopia development in chicks, and some other animal models, can be suppressed by rearing under near-monochromatic, short-wavelength blue light. We aimed to determine whether similar effects could be achieved using glass filters that transmit a broader range of short and middle wavelengths.

Methods

On day 6 or 7 post-hatch, 169 chicks were assigned to one of three monocular lens conditions (−10 D, +10 D, plano) and reared for 7 or 10 days under one of four 201-lux lighting conditions: (1) B410 long-wavelength–filtered light, (2) B460 long-wavelength–filtered light, (3) Y48 short-wavelength–filtered light, or (4) HA50 broadband light.

Results

At 7 days, B410 (but not B460) long-wavelength–filtered light had significantly inhibited negative lens induced axial growth relative to Y48 short-wavelength–filtered light (mean difference in experimental eye = −0.249 mm; P = 0.006) and HA50 broadband light (mean difference = −0.139 mm; P = 0.038). B410 filters also inhibited the negative lens-induced increase in vitreous chamber depth relative to all other filter conditions. Corresponding changes in refraction did not occur, and biometric measurements in a separate cohort of chicks suggested that the axial dimension changes were transient and not maintained at 10 days.

Conclusions

Chromatic effects on eye growth can be achieved using filters that transmit a broad range of wavelengths even in the presence of strong cues for myopia development.

Translational Relevance

Broad-wavelength filters that provide a more “naturalistic” visual experience relative to monochromatic light have potential to alter myopia development, although the effects shown here were modest and transient and require exploration in further species.

Recreated Ancestral Opsin Associated with Marine to Freshwater Croaker Invasion Reveals Kinetic and Spectral Adaptation

Rhodopsin, the light-sensitive visual pigment expressed in rod photoreceptors, is specialized for vision in dim-light environments. Aquatic environments are particularly challenging for vision due to the spectrally dependent attenuation of light, which can differ greatly in marine and freshwater systems. Among fish lineages that have successfully colonized freshwater habitats from ancestrally marine environments, croakers are known as highly visual benthic predators. In this study, we isolate rhodopsins from a diversity of freshwater and marine croakers and find that strong positive selection in rhodopsin is associated with a marine to freshwater transition in South American croakers. In order to determine if this is accompanied by significant shifts in visual abilities, we resurrected ancestral rhodopsin sequences and tested the experimental properties of ancestral pigments bracketing this transition using in vitro spectroscopic assays. We found the ancestral freshwater croaker rhodopsin is redshifted relative to its marine ancestor, with mutations that recapitulate ancestral amino acid changes along this transitional branch resulting in faster kinetics that are likely to be associated with more rapid dark adaptation. This could be advantageous in freshwater due to the redshifted spectrum and relatively narrow interface and frequent transitions between bright and dim-light environments. This study is the first to experimentally demonstrate that positively selected substitutions in ancestral visual pigments alter protein function to freshwater visual environments following a transition from an ancestrally marine state and provides insight into the molecular mechanisms underlying some of the physiological changes associated with this major habitat transition.

The evolution of red color vision is linked to coordinated rhodopsin tuning in lycaenid butterflies

Color vision has evolved multiple times in both vertebrates and invertebrates and is largely determined by the number and variation in spectral sensitivities of distinct opsin subclasses. However, because of the difficulty of expressing long-wavelength (LW) invertebrate opsins in vitro, our understanding of the molecular basis of functional shifts in opsin spectral sensitivities has been biased toward research primarily in vertebrates. This has restricted our ability to address whether invertebrate Gq protein-coupled opsins function in a novel or convergent way compared to vertebrate Gt opsins. Here we develop a robust heterologous expression system to purify invertebrate rhodopsins, identify specific amino acid changes responsible for adaptive spectral tuning, and pinpoint how molecular variation in invertebrate opsins underlie wavelength sensitivity shifts that enhance visual perception. By combining functional and optophysiological approaches, we disentangle the relative contributions of lateral filtering pigments from red-shifted LW and blue short-wavelength opsins expressed in distinct photoreceptor cells of individual ommatidia. We use in situ hybridization to visualize six ommatidial classes in the compound eye of a lycaenid butterfly with a four-opsin visual system. We show experimentally that certain key tuning residues underlying green spectral shifts in blue opsin paralogs have evolved repeatedly among short-wavelength opsin lineages. Taken together, our results demonstrate the interplay between regulatory and adaptive evolution at multiple Gq opsin loci, as well as how coordinated spectral shifts in LW and blue opsins can act together to enhance insect spectral sensitivity at blue and red wavelengths for visual performance adaptation.

A Review on Fish Sensory Systems and Amazon Water Types With Implications to Biodiversity

The Amazon has the highest richness of freshwater organisms in the world, which has led to a multitude of hypotheses on the mechanisms that generated this biodiversity. However, most of these hypotheses focus on the spatial distance of populations, a framework that fails to provide an explicit mechanism of speciation. Ecological conditions in Amazon freshwaters can be strikingly distinct, as it has been recognized since Alfred Russel Wallace’s categorization into black, white, and blue (= clear) waters. Water types reflect differences in turbidity, dissolved organic matter, electrical conductivity, pH, amount of nutrients and lighting environment, characteristics that directly affect the sensory abilities of aquatic organisms. Since natural selection drives evolution of sensory systems to function optimally according to environmental conditions, the sensory systems of Amazon freshwater organisms are expected to vary according to their environment. When differences in sensory systems affect chances of interbreeding between populations, local adaptations may result in speciation. Here, we briefly present the limnologic characteristics of Amazonian water types and how they are expected to influence photo-, chemical-, mechano-, and electro-reception of aquatic organisms, focusing on fish. We put forward that the effect of different water types on the adaptation of sensory systems is an important mechanism that contributed to the evolution of fish diversity. We point toward underexplored research perspectives on how divergent selection may act on sensory systems and thus contribute to the origin and maintenance of the biodiversity of Amazon aquatic environments.

Genetic variability of the sws1 cone opsin gene among New World monkeys

Vision is a major sense for Primates and the ability to perceive colors has great importance for the species ecology and behavior. Visual processing begins with the activation of the visual opsins in the retina, and the spectral absorption peaks are highly variable among species. In most Primates, LWS/MWS opsins are responsible for sensitivity to long/middle wavelengths within the visible light spectrum, and SWS1 opsins provide sensitivity to short wavelengths, in the violet region of the spectrum. In this study, we aimed to investigate the genetic variation on the sws1 opsin gene of New World monkeys (NWM) and search for amino acid substitutions that might be associated with the different color vision phenotypes described for a few species. We sequenced the exon 1 of the sws1 opsin gene of seven species from the families Callitrichidae, Cebidae, and Atelidae, and searched for variation at the spectral tuning sites 46, 49, 52, 86, 90, 93, 114, 116, and 118. Among the known spectral tuning sites, only residue 114 was variable. To investigate whether other residues have a functional role in the SWS1 absorption peak, we performed computational modeling of wild-type SWS1 and mutants A50I and A50V, found naturally among the species investigated. Although in silico analysis did not show any visible effect caused by these substitutions, it is possible that interactions of residue 50 with other sites might have some effect in the spectral shifts in the order of ~14 nm, found among the NWM. We also performed phylogenetic reconstruction of the sws1 gene, which partially recovered the species phylogeny. Further studies will be important to uncover the mutations responsible for the phenotypic variability of the SWS1 of NWM, and how spectral tuning may be associated with specific ecological features such as preferred food items and habitat use.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

Visual pigment evolution in Characiformes: The dynamic interplay of teleost whole-genome duplication, surviving opsins and spectral tuning

Vision represents an excellent model for studying adaptation, given the genotype-to-phenotype map that has been characterized in a number of taxa. Fish possess a diverse range of visual sensitivities and adaptations to underwater light, making them an excellent group to study visual system evolution. In particular, some speciose but understudied lineages can provide a unique opportunity to better understand aspects of visual system evolution such as opsin gene duplication and neofunctionalization. In this study, we showcase the visual system evolution of neotropical Characiformes and the spectral tuning mechanisms they exhibit to modulate their visual sensitivities. Such mechanisms include gene duplications and losses, gene conversion, opsin amino acid sequence and expression variation, and A₁ /A₂ -chromophore shifts. The Characiforms we studied utilize three cone opsin classes (SWS2, RH2, LWS) and a rod opsin (RH1). However, the characiform's entire opsin gene repertoire is a product of dynamic evolution by opsin gene loss (SWS1, RH2) and duplication (LWS, RH1). The LWS- and RH1-duplicates originated from a teleost specific whole-genome duplication as well as characiform-specific duplication events. Both LWS-opsins exhibit gene conversion and, through substitutions in key tuning sites, one of the LWS-paralogues has acquired spectral sensitivity to green light. These sequence changes suggest reversion and parallel evolution of key tuning sites. Furthermore, characiforms' colour vision is based on the expression of both LWS-paralogues and SWS2. Finally, we found interspecific and intraspecific variation in A₁ /A₂ -chromophores proportions, correlating with the light environment. These multiple mechanisms may be a result of the diverse visual environments where Characiformes have evolved.

Adaptations and evolutionary trajectories of the snake rod and cone photoreceptors

Most vertebrates have duplex retinas, with two classes of photoreceptors, rods and cones. In the group of Snakes, however, distinct patterns of retinal morphology are associated with transitions between diurnal-nocturnal habits and reflect important adaptations of their visual system. Pure-cone, pure-rod and duplex retinas were described in different species, and this variability led Gordon Walls (1934) to formulate the transmutation theory, which suggests that rods and cones are not fixed entities, but can assume transitional states. Three opsin genes are expressed in retinas of most snake species, lws, rh1, and sws1, and recent studies have shown that the rhodopsin gene, rh1, is expressed in pure-cone retinas of diurnal snakes. This expression raised many questions about the nature of transmutation and functional aspects of the rhodopsin in a cone-like photoreceptor. Extreme differences in the retinal architecture of diurnal and nocturnal snakes also highlight the complexity of adaptations of their visual structures, which might have contributed to the adaptive radiation of this group and will be discussed in this review.

Color discrimination assessment in patients with hypothyroidism using the Farnsworth-Munsell 100 hue test

There is evidence in the literature that hypofunction of the thyroid gland (hypothyroidism) affects color vision in rodents by influencing the production of the visual pigment opsin. The effect of hypothyroidism on color vision in humans has not been examined in any great detail. In this cross-sectional study we evaluated color discrimination using the Farnsworth-Munsell 100 hue test (FM-100 test) in 25 individuals with pre-treatment hypothyroidism (mean age ${38}\;{\pm }\;{9}.{2}\;\text{yr}$38±9.2yr), and a control euthyroid group, ${ n} ={26}$n=26 (mean age ${39.6}\;{\pm }\;{8}.{4}\;\text{yr}$39.6±8.4yr). There was no statistically significant difference in the total error score ($\surd{\text{TES}}$√TES) between the groups, but the hypothyroid group had a significantly greater partial error score ($\surd{\text{PES}}$√PES) along the blue-yellow (B-Y) axis compared to the red-green (R-G) axis. No statistically significant differences in B-Y and R-G PES were observed in the control group. This study shows that hypothyroidism affects color vision in humans, causing significant impairment in the B-Y color subsystem.

Viewing Nuclear Architecture through the Eyes of Nocturnal Mammals

The cell nucleus is a remarkably well-organized organelle with membraneless but distinct compartments of various functions. The largest of them, euchromatin and heterochromatin, are spatially segregated in such a way that the transcriptionally active genome occupies the nuclear interior, whereas silent genomic loci are preferentially associated with the nuclear envelope. This rule is broken by rod photoreceptor cells of nocturnal mammals, in which the two major compartments have inverted positions. The inversion and dense compaction of heterochromatin converts these nuclei into microlenses that focus light and facilitate nocturnal vision. As is often the case in biology, when a mutation helps to understand normal processes and structures, inverted nuclei have served as a tool to unravel general principles of nuclear organization, including mechanisms of heterochromatin tethering to the nuclear envelope, autonomous behavior of small genomic segments, and euchromatin-heterochromatin segregation.

Diurnal variation in opsin expression and common housekeeping genes necessitates comprehensive normalization methods for quantitative real-time PCR analyses

To determine the visual sensitivities of an organism of interest, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) is often used to quantify expression of the light-sensitive opsins in the retina. While qRT-PCR is an affordable, high-throughput method for measuring expression, it comes with inherent normalization issues that affect the interpretation of results, especially as opsin expression can vary greatly based on developmental stage, light environment or diurnal cycles. We tested for diurnal cycles of opsin expression over a period of 24 hr at 1-hr increments and examined how normalization affects a data set with fluctuating expression levels using qRT-PCR and transcriptome data from the retinae of the cichlid Pelmatolapia mariae. We compared five methods of normalizing opsin expression relative to (a) the average of three stably expressed housekeeping genes (Ube2z, EF1-α and β-actin), (b) total RNA concentration, (c) GNAT2, (the cone-specific subunit of transducin), (d) total opsin expression and (e) only opsins expressed in the same cone type. Normalizing by proportion of cone type produced the least variation and would be best for removing time-of-day variation. In contrast, normalizing by housekeeping genes produced the highest daily variation in expression and demonstrated that the peak of cone opsin expression was in the late afternoon. A weighted correlation network analysis showed that the expression of different cone opsins follows a very similar daily cycle. With the knowledge of how these normalization methods affect opsin expression data, we make recommendations for designing sampling approaches and quantification methods based upon the scientific question being examined.

Retinal Organ Cultures as Alternative Research Models

Ex vivo organ cultures represent unique research models, as they combine the advantages of cell cultures with those of animal models. Being able to mimic in vivo situations through the use of organ cultures provides an excellent opportunity to investigate cellular processes, molecular pathways and cell–cell interactions, as well as structural and synaptic organisation. Human and animal organ cultures are now well established and comprise sensitive, easy-to-manipulate experimental systems that raise minimal ethical concerns. The eye, in particular, is a very complex organ that is not easy to reproduce in vitro. However, a lot of research has been dedicated to the development of suitable ocular organ cultures. This review covers the various ex vivo retinal organ culture systems available for use in ophthalmology research and compares them with commonly used animal models. In particular, bovine and porcine retinal organ culture systems are described, because the size, anatomy, physiology and vessel morphology of bovine and porcine eyes are similar to the human eye in an undisputed way, thus making them good models. In addition, these animals are widely used by the food industry and the eyes are considered surplus material. A short overview of murine, rat, rabbit, cat, canine and simian retinal organ cultures is also provided.

Photoreceptors morphology and genetics of the visual pigments of Bothrops jararaca and Crotalus durissus terrificus (Serpentes, Viperidae)

Snakes inhabit a great variety of habitats, whose spectral quality of light may vary a lot and influence specific adaptations of their visual system. In this study, we investigated the genetics of the visual opsins and the morphology of retinal photoreceptors, of two nocturnal snakes from the Viperidae family, Bothrops jararaca and Crotalus durissus terrificus, which inhabit preferentially the Atlantic Rain Forest and the Brazilian Savannah, respectively. Total RNA was extracted from homogenized retinas and converted to cDNA. The opsin genes expressed in snake retinas, LWS, RH1, and SWS1, were amplified by polymerase chain reactions (PCRs) and sequenced. The absorption peak (λ_max) of the opsins were estimated based on amino acids located at specific spectral tuning sites. Photoreceptor cell populations were analyzed using immunohistochemistry with anti-opsin antibodies. Results showed the same morphological cell populations and same opsins absorption peaks, in both viperid species: double and single cones with LWS photopigment and λ_max at ∼555 nm; single cones with SWS1 photopigment and λ_max at ∼360 nm; and rods with the rhodopsin RH1 photopigment and λ_max at ∼500 nm. The results indicate adaptations to nocturnal habit in both species despite the differences in habitat, and the possibility of a dichromatic color vision at photopic conditions.

As Blind as a Bat? Opsin Phylogenetics Illuminates the Evolution of Color Vision in Bats

Through their unique use of sophisticated laryngeal echolocation bats are considered sensory specialists amongst mammals and represent an excellent model in which to explore sensory perception. Although several studies have shown that the evolution of vision is linked to ecological niche adaptation in other mammalian lineages, this has not yet been fully explored in bats. Recent molecular analysis of the opsin genes, which encode the photosensitive pigments underpinning color vision, have implicated high-duty cycle (HDC) echolocation and the adoption of cave roosting habits in the degeneration of color vision in bats. However, insufficient sampling of relevant taxa has hindered definitive testing of these hypotheses. To address this, novel sequence data was generated for the SWS1 and MWS/LWS opsin genes and combined with existing data to comprehensively sample species representing diverse echolocation types and niches (SWS1 n = 115; MWS/LWS n = 45). A combination of phylogenetic analysis, ancestral state reconstruction, and selective pressure analyses were used to reconstruct the evolution of these visual pigments in bats and revealed that although both genes are evolving under purifying selection in bats, MWS/LWS is highly conserved but SWS1 is highly variable. Spectral tuning analyses revealed that MWS/LWS opsin is tuned to a long wavelength, 555-560 nm in the bat ancestor and the majority of extant taxa. The presence of UV vision in bats is supported by our spectral tuning analysis, but phylogenetic analyses demonstrated that the SWS1 opsin gene has undergone pseudogenization in several lineages. We do not find support for a link between the evolution of HDC echolocation and the pseudogenization of the SWS1 gene in bats, instead we show the SWS1 opsin is functional in the HDC echolocator, Pteronotus parnellii. Pseudogenization of the SWS1 is correlated with cave roosting habits in the majority of pteropodid species. Together these results demonstrate that the loss of UV vision in bats is more widespread than was previously considered and further elucidate the role of ecological niche specialization in the evolution of vision in bats.

Whole-genome sequences of Malawi cichlids reveal multiple radiations interconnected by gene flow

The hundreds of cichlid fish species in Lake Malawi constitute the most extensive recent vertebrate adaptive radiation. Here we characterize its genomic diversity by sequencing 134 individuals covering 73 species across all major lineages. The average sequence divergence between species pairs is only 0.1-0.25%. These divergence values overlap diversity within species, with 82% of heterozygosity shared between species. Phylogenetic analyses suggest that diversification initially proceeded by serial branching from a generalist Astatotilapia-like ancestor. However, no single species tree adequately represents all species relationships, with evidence for substantial gene flow at multiple times. Common signatures of selection on visual and oxygen transport genes shared by distantly related deep-water species point to both adaptive introgression and independent selection. These findings enhance our understanding of genomic processes underlying rapid species diversification, and provide a platform for future genetic analysis of the Malawi radiation.

Spectral sensitivity in ray-finned fishes: diversity, ecology and shared descent

A major goal of sensory ecology is to identify factors that underlie sensory-trait variation. One open question centers on why fishes show the greatest diversity among vertebrates in their capacity to detect color (i.e. spectral sensitivity). Over the past several decades, λ_max values (photoreceptor class peak sensitivity) and chromacy (photoreceptor class number) have been cataloged for hundreds of fish species, yet the ecological basis of this diversity and the functional significance of high chromacy levels (e.g. tetra- and pentachromacy) remain unclear. In this study, we examined phylogenetic, physiological and ecological patterns of spectral sensitivity of ray-finned fishes (Actinoptergyii) via a meta-analysis of data compiled from 213 species. Across the fishes sampled, our results indicate that trichromacy is most common, ultraviolet λ_max values are not found in monochromatic or dichromatic species, and increasing chromacy, including from tetra- to pentachromacy, significantly increases spectral sensitivity range. In an ecological analysis, multivariate phylogenetic latent liability modeling was performed to analyze correlations between chromacy and five hypothesized predictors (depth, habitat, diet, body coloration, body size). In a model not accounting for phylogenetic relatedness, each predictor with the exception of habitat significantly correlated with chromacy: a positive relationship in body color and negative relationships with body size, diet and depth. However, after phylogenetic correction, the only remaining correlated predictor was depth. The findings of this study indicate that phyletic heritage and depth are important factors in fish spectral sensitivity and impart caution about excluding phylogenetic comparative methods in studies of sensory trait variation.

Shark genomes provide insights into elasmobranch evolution and the origin of vertebrates

Modern cartilaginous fishes are divided into elasmobranchs (sharks, rays and skates) and chimaeras, and the lack of established whole-genome sequences for the former has prevented our understanding of early vertebrate evolution and the unique phenotypes of elasmobranchs. Here we present de novo whole-genome assemblies of brownbanded bamboo shark and cloudy catshark and an improved assembly of the whale shark genome. These relatively large genomes (3.8-6.7 Gbp) contain sparse distributions of coding genes and regulatory elements and exhibit reduced molecular evolutionary rates. Our thorough genome annotation revealed Hox C genes previously hypothesized to have been lost, as well as distinct gene repertories of opsins and olfactory receptors that would be associated with adaptation to unique underwater niches. We also show the early establishment of the genetic machinery governing mammalian homoeostasis and reproduction at the jawed vertebrate ancestor. This study, supported by genomic, transcriptomic and epigenomic resources, provides a foundation for the comprehensive, molecular exploration of phenotypes unique to sharks and insights into the evolutionary origins of vertebrates.

Seasonal changes in color perception

In temperate zones, organisms experience dynamic fluctuations in environment including changes in color. To cope with such seasonal changes in the environment, organisms adapt their physiology and behavior. Although color perception has been believed to be fixed throughout life, there is increasing evidence for the alteration in opsin gene expression induced by environmental stimuli in a number of animals. Very recently, dynamic seasonal plasticity in color perception has been reported in the seasonally breeding medaka fish. Interestingly, seasonal changes in human color perception have also been reported. Therefore, plasticity of color perception, induced by environmental stimuli, might be a common phenomenon across various species.

Adaptive genomic evolution of opsins reveals that early mammals flourished in nocturnal environments

BACKGROUND

Based on evolutionary patterns of the vertebrate eye, Walls (1942) hypothesized that early placental mammals evolved primarily in nocturnal habitats. However, not only Eutheria, but all mammals show photic characteristics (i.e. dichromatic vision, rod-dominated retina) suggestive of a scotopic eye design.

RESULTS

Here, we used integrative comparative genomic and phylogenetic methodologies employing the photoreceptive opsin gene family in 154 mammals to test the likelihood of a nocturnal period in the emergence of all mammals. We showed that mammals possess genomic patterns concordant with a nocturnal ancestry. The loss of the RH2, VA, PARA, PARIE and OPN4x opsins in all mammals led us to advance a probable and most-parsimonious hypothesis of a global nocturnal bottleneck that explains the loss of these genes in the emerging lineage (> > 215.5 million years ago). In addition, ancestral character reconstruction analyses provided strong evidence that ancestral mammals possessed a nocturnal lifestyle, ultra-violet-sensitive vision, low visual acuity and low orbit convergence (i.e. panoramic vision).

CONCLUSIONS

Overall, this study provides insight into the evolutionary history of the mammalian eye while discussing important ecological aspects of the photic paleo-environments ancestral mammals have occupied.

Comparative visual ecology of cephalopods from different habitats

Previous investigations of vision and visual pigment evolution in aquatic predators have focused on fish and crustaceans, generally ignoring the cephalopods. Since the first cephalopod opsin was sequenced in late 1980s, we now have data on over 50 cephalopod opsins, prompting this functional and phylogenetic examination. Much of this data does not specifically examine the visual pigment spectral absorbance position (λ_max) relative to environment or lifestyle, and cephalopod opsin functional adaptation and visual ecology remain largely unknown. Here we introduce a new protocol for photoreceptor microspectrophotometry (MSP) that overcomes the difficulty of bleaching the bistable visual pigment and that reveals eight coastal coleoid cephalopods to be monochromatic with λ_max varying from 484 to 505 nm. A combination of current MSP results, the λ_max values previously characterized using cephalopod retinal extracts (467–500 nm) and the corresponding opsin phylogenetic tree were used for systematic comparisons with an end goal of examining the adaptations of coleoid visual pigments to different light environments. Spectral tuning shifts are described in response to different modes of life and light conditions. A new spectral tuning model suggests that nine amino acid substitution sites may determine the direction and the magnitude of spectral shifts.

The genomic basis of cichlid fish adaptation within the deepwater "twilight zone" of Lake Malawi

Deepwater environments are characterized by low levels of available light at narrow spectra, great hydrostatic pressure, and low levels of dissolved oxygen—conditions predicted to exert highly specific selection pressures. In Lake Malawi over 800 cichlid species have evolved, and this adaptive radiation extends into the “twilight zone” below 50 m. We use population‐level RAD‐seq data to investigate whether four endemic deepwater species (Diplotaxodon spp.) have experienced divergent selection within this environment. We identify candidate genes including regulators of photoreceptor function, photopigments, lens morphology, and haemoglobin, many not previously implicated in cichlid adaptive radiations. Colocalization of functionally linked genes suggests coadapted “supergene” complexes. Comparisons of Diplotaxodon to the broader Lake Malawi radiation using genome resequencing data revealed functional substitutions and signatures of positive selection in candidate genes. Our data provide unique insights into genomic adaptation within deepwater habitats, and suggest genome‐level specialization for life at depth as an important process in cichlid radiation.

Behavioral color vision in a cichlid fish: Metriaclima benetos

Color vision is the capacity to discriminate color regardless of brightness. It is essential for many fish species as they rely on color discrimination for numerous ecological tasks. The study of color vision is important because it can unveil the mechanisms that shape coloration patterns, visual system sensitivities and, hence, visual signals. In order to better understand the mechanisms underlying color vision, an integrative approach is necessary. This usually requires combining behavioral, physiological and genetic experiments with quantitative modeling, resulting in a distinctive characterization of the visual system. Here, we provide new data on the color vision of a rock-dwelling cichlid from Lake Malawi: Metriaclima benetos. For this study we used a behavioral approach to demonstrate color vision through classical conditioning, complemented with modeling of color vision to estimate color contrast. For our experiments we took into account opsin coexpression and considered whether cichlids exhibit a dichromatic or a trichromatic visual system. Behavioral experiments confirmed color vision in M. benetos; most fish were significantly more likely to choose the trained over the distracter stimuli, irrespective of brightness. Our results are supported by visual modeling that suggests that cichlids are trichromats and achieve color vision through color opponency mechanisms, which are a result of three different photoreceptor channels. Our analyses also suggest that opsin coexpression can negatively affect perceived color contrast. This study is particularly relevant for research on the cichlid lineage because cichlid visual capabilities and coloration patterns are implicated in their adaptive radiation.

Losing focus: how lens position and viewing angle affect the function of multifocal lenses in fishes

Light rays of different wavelengths are focused at different distances when they pass through a lens (longitudinal chromatic aberration [LCA]). For animals with color vision this can pose a serious problem, because in order to perceive a sharp image the rays must be focused at the shallow plane of the photoreceptor's outer segments in the retina. A variety of fish and tetrapods have been found to possess multifocal lenses, which correct for LCA by assigning concentric zones to correctly focus specific wavelengths. Each zone receives light from a specific beam entrance position (BEP) (the lateral distance between incoming light and the center of the lens). Any occlusion of incoming light at specific BEPs changes the composition of the wavelengths that are correctly focused on the retina. Here, we calculated the effect of lens position relative to the plane of the iris and light entering the eye at oblique angles on how much of the lens was involved in focusing the image on the retina (measured as the availability of BEPs). We used rotational photography of fish eyes and mathematical modeling to quantify the degree of lens occlusion. We found that, at most lens positions and viewing angles, there was a decrease of BEP availability and in some cases complete absence of some BEPs. Given the implications of these effects on image quality, we postulate that three morphological features (aphakic spaces, curvature of the iris, and intraretinal variability in spectral sensitivity) may, in part, be adaptations to mitigate the loss of spectral image quality in the periphery of the eyes of fishes.

Out of the blue: adaptive visual pigment evolution accompanies Amazon invasion

Incursions of marine water into South America during the Miocene prompted colonization of freshwater habitats by ancestrally marine species and present a unique opportunity to study the molecular evolution of adaptations to varying environments. Freshwater and marine environments are distinct in both spectra and average intensities of available light. Here, we investigate the molecular evolution of rhodopsin, the photosensitive pigment in the eye that activates in response to light, in a clade of South American freshwater anchovies derived from a marine ancestral lineage. Using likelihood-based comparative sequence analyses, we found evidence for positive selection in the rhodopsin of freshwater anchovy lineages at sites known to be important for aspects of rhodopsin function such as spectral tuning. No evidence was found for positive selection in marine lineages, nor in three other genes not involved in vision. Our results suggest that an increased rate of rhodopsin evolution was driven by diversification into freshwater habitats, thereby constituting a rare example of molecular evolution mirroring large-scale palaeogeographic events.

Diverse Distributions of Extraocular Opsins in Crustaceans, Cephalopods, and Fish

Non-visual and extraocular photoreceptors are common among animals, but current understanding linking molecular pathways to physiological function of these receptors is lacking. Opsin diversity in extraocular tissues suggests that many putative extraocular photoreceptors utilize the "visual" phototransduction pathway-the same phototransduction pathway as photoreceptors within the retina dedicated to light detection for image sensing. Here, we provide a brief overview of the current understanding of non-visual and extraocular photoreceptors, and contribute a synopsis of several novel putative extraocular photoreceptors that use both visual and non-visual phototransduction pathways. Crayfish, cephalopods, and flat fish express opsins in diverse tissues, suggesting the presence of extraocular photoreceptors. In most cases, we find that these animals use the same phototransduction pathway that is utilized in the retinas for image-formation. However, we also find the presence of non-visual phototransduction components in the skin of flounders. Our evidence suggests that extraocular photoreceptors may employ a number of phototransduction pathways that do not appear to correlate with purpose or location of the photoreceptor.

Spatial resolving power and spectral sensitivity of the saltwater crocodile, Crocodylus porosus, and the freshwater crocodile, Crocodylus johnstoni

Crocodilians are apex amphibious predators that occupy a range of tropical habitats. In this study, we examined whether their semi-aquatic lifestyle and ambush hunting mode are reflected in specific adaptations in the peripheral visual system. Design-based stereology and microspectrophotometry were used to assess spatial resolving power and spectral sensitivity of saltwater (Crocodylus porosus) and freshwater crocodiles (Crocodylus johnstoni). Both species possess a foveal streak that spans the naso-temporal axis and mediates high spatial acuity across the central visual field. The saltwater crocodile and freshwater crocodile have a peak spatial resolving power of 8.8 and 8.0 cycles deg−1, respectively. Measurement of the outer segment dimensions and spectral absorbance revealed five distinct photoreceptor types consisting of three single cones, one twin cone and a rod. The three single cones (saltwater/freshwater crocodile) are violet (424/426 nm λmax), green (502/510 nm λmax) and red (546/554 nm λmax) sensitive, indicating the potential for trichromatic colour vision. The visual pigments of both members of the twin cones have the same λmax as the red-sensitive single cone and the rod has a λmax at 503/510 nm (saltwater/freshwater). The λmax values of all types of visual pigment occur at longer wavelengths in the freshwater crocodile compared with the saltwater crocodile. Given that there is a greater abundance of long wavelength light in freshwater compared with a saltwater environment, the photoreceptors would be more effective at detecting light in their respective habitats. This suggests that the visual systems of both species are adapted to the photic conditions of their respective ecological niche.

A comparative study of rhodopsin function in the great bowerbird (Ptilonorhynchus nuchalis): Spectral tuning and light-activated kinetics

Rhodopsin is the visual pigment responsible for initiating the phototransduction cascade in vertebrate rod photoreceptors. Although well-characterized in a few model systems, comparative studies of rhodopsin function, particularly for nonmammalian vertebrates are comparatively lacking. Bowerbirds are rare among passerines in possessing a key substitution, D83N, at a site that is otherwise highly conserved among G protein-coupled receptors. While this substitution is present in some dim-light adapted vertebrates, often accompanying another unusual substitution, A292S, its functional relevance in birds is uncertain. To investigate functional effects associated with these two substitutions, we use the rhodopsin gene from the great bowerbird (Ptilonorhynchus nuchalis) as a background for site-directed mutagenesis, in vitro expression and functional characterization. We also mutated these sites in two additional rhodopsins that do not naturally possess N83, chicken and bovine, for comparison. Both sites were found to contribute to spectral blue-shifts, but had opposing effects on kinetic rates. Substitutions at site 83 were found to primarily affect the kinetics of light-activated rhodopsin, while substitutions at site 292 had a larger impact on spectral tuning. The contribution of substitutions at site 83 to spectral tuning in particular depended on genetic background, but overall, the effects of substitutions were otherwise surprisingly additive, and the magnitudes of functional shifts were roughly similar across all three genetic backgrounds. By employing a comparative approach with multiple species, our study provides new insight into the joint impact of sites 83 and 292 on rhodopsin structure-function as well as their evolutionary significance for dim-light vision across vertebrates.

The genome of the miiuy croaker reveals well-developed innate immune and sensory systems

The miiuy croaker, Miichthys miiuy, is a representative Sciaenidae known for its exceptionally large otoliths. This species mainly inhabits turbid aquatic environments with mud to sandy mud bottoms. However, the characteristics of the immune system of this organism and its specific aquatic environment adaptations are poorly understood. Thus, we present a high-quality draft genome of miiuy croaker. The expansions of several gene families which are critical for the fish innate immune system were identified. Compared with the genomes of other fishes, some changes have occurred in the miiuy croaker sensory system including modification of vision and expansion of taste and olfaction receptors. These changes allow miiuy croaker to adapt to the environment during the long-term natural selection. The genome of miiuy croaker may elucidate its relatively well-developed immune defense and provide an adaptation model of the species thriving in turbid deep aquatic environments.

Sexual dimorphism in the compound eye of Heliconius erato: a nymphalid butterfly with at least five spectral classes of photoreceptor

Most butterfly families expand the number of spectrally-distinct photoreceptors in their compound eye by opsin gene duplications together with lateral filter pigments, however most nymphalid genera have limited diversity, with only three or four spectral types of photoreceptor. Here we examine the spatial pattern of opsin expression and photoreceptor spectral sensitivities in Heliconius erato, a nymphalid with duplicate ultraviolet opsin genes, UVRh1 and UVRh2. We find that the H. erato compound eye is sexually dimorphic. Females express the two UV opsin proteins in separate photoreceptors, but males do not express UVRh1. Intracellular recordings confirmed that females have three short wavelength-sensitive photoreceptors (λmax=356 nm, ∼390 nm and 470 nm), while males have two (λmax=390 nm and ∼470 nm). We also found two long wavelength-sensitive photoreceptors (green, λmax ∼555 nm, and red, λmax ∼600 nm), which express the same LW opsin. The red cell's shifted sensitivity is probably due to perirhabdomal filtering pigments. Sexual dimorphism of the UV-absorbing rhodopsins may reflect the females' need to discriminate conspecifics from co-mimics. Red-green color vision may be used to detect differences in red coloration on Heliconius wings, or for host-plant identification. Among nymphalids so far investigated, only H. erato is known to possess five spectral classes of photoreceptor; sexual dimorphism of the eye via suppression of one class of opsin (here UVRh1 in males) has not—to our knowledge—been reported in any animal.

Spectral Tuning in the Eyes of Deep-Sea Lanternfishes (Myctophidae): A Novel Sexually Dimorphic Intra-Ocular Filter

Deep-sea fishes possess several adaptations to facilitate vision where light detection is pushed to its limit. Lanternfishes (Myctophidae), one of the world's most abundant groups of mesopelagic fishes, possess a novel and unique visual specialisation, a sexually dimorphic photostable yellow pigmentation, constituting the first record of a visual sexual dimorphism in any non-primate vertebrate. The topographic distribution of the yellow pigmentation across the retina is species specific, varying in location, shape and size. Spectrophotometric analyses reveal that this new retinal specialisation differs between species in terms of composition and acts as a filter, absorbing maximally between 356 and 443 nm. Microspectrophotometry and molecular analyses indicate that the species containing this pigmentation also possess at least 2 spectrally distinct rod visual pigments as a result of a duplication of the Rh1 opsin gene. After modelling the effect of the yellow pigmentation on photoreceptor spectral sensitivity, we suggest that this unique specialisation acts as a filter to enhance contrast, thereby improving the detection of bioluminescent emissions and possibly fluorescence in the extreme environment of the deep sea. The fact that this yellow pigmentation is species specific, sexually dimorphic and isolated within specific parts of the retina indicates an evolutionary pressure to visualise prey/predators/mates in a particular part of each species' visual field.

The effect of hypothyroidism on color contrast sensitivity: a prospective study

BACKGROUND

Thyroid hormone has been shown to control retinal cone opsin expression, the protein of color vision, in adult rodents.

OBJECTIVES

The aim of this study was to evaluate the effect of hypothyroidism on color contrast sensitivity in adult overt hypothyroid patients.

METHODS

Thirty-eight overt hypothyroid (31 females, 7 males) subjects and 20 euthyroid (16 females, 4 males) controls were studied prospectively. Color vision examination was performed by Chromatest, a software program analyzing the tritan (blue-yellow) color contrast threshold (tritan CCT) and protan (red-green) color contrast threshold (protan CCT). Color contrast sensitivity analyses of hypothyroid subjects were performed on admission and after L-thyroxine treatment when biochemical euthyroidism was achieved.

RESULTS

After a median period of 90 (90-210) days, 24 (19 females, 5 males) patients were euthyroid and eligible for a second color vision examination. Baseline tritan CCT and protan CCT values were significantly higher in the hypothyroid group compared to euthyroid controls, which clinically translates into impaired color contrast sensitivity (p < 0.001 and p < 0.001, respectively). There was a significant decrease in tritan CCT (p = 0.002) and protan CCT (p < 0.001) values in the hypothyroid group after euthyroidism was achieved, which denotes improvement in color contrast sensitivity.

CONCLUSIONS

It is a novel finding of the current study that color contrast sensitivity is impaired in hypothyroidism and significantly improves after euthyroidism is achieved.

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.

Ancestral duplications and highly dynamic opsin gene evolution in percomorph fishes

Single-gene and whole-genome duplications are important evolutionary mechanisms that contribute to biological diversification by launching new genetic raw material. For example, the evolution of animal vision is tightly linked to the expansion of the opsin gene family encoding light-absorbing visual pigments. In teleost fishes, the most species-rich vertebrate group, opsins are particularly diverse and key to the successful colonization of habitats ranging from the bioluminescence-biased but basically dark deep sea to clear mountain streams. In this study, we report a previously unnoticed duplication of the violet-blue short wavelength-sensitive 2 (SWS2) opsin, which coincides with the radiation of highly diverse percomorph fishes, permitting us to reinterpret the evolution of this gene family. The inspection of close to 100 fish genomes revealed that, triggered by frequent gene conversion between duplicates, the evolutionary history of SWS2 is rather complex and difficult to predict. Coincidentally, we also report potential cases of gene resurrection in vertebrate opsins, whereby pseudogenized genes were found to convert with their functional paralogs. We then identify multiple novel amino acid substitutions that are likely to have contributed to the adaptive differentiation between SWS2 copies. Finally, using the dusky dottyback Pseudochromis fuscus, we show that the newly discovered SWS2A duplicates can contribute to visual adaptation in two ways: by gaining sensitivities to different wavelengths of light and by being differentially expressed between ontogenetic stages. Thus, our study highlights the importance of comparative approaches in gaining a comprehensive view of the dynamics underlying gene family evolution and ultimately, animal diversification.

Correlation between dichromatic colour vision and jumping performance in horses

There is general agreement that horses have dichromatic colour vision with similar capabilities to human beings with red-green colour deficiencies. However, whether colour perception has an impact on equine jumping performance and how pronounced the colour stimulus might be for a horse is unknown. The present study investigated the relationship between the colour of the fences (blue or green) and the show jumping performance of 20 horses ridden by two riders using an indoor and outdoor set of green and blue fences. In the indoor arena, significantly more touches and faults were made on blue fences in comparison to green fences (median difference of 2.5 bars). When only touched bars were included, a significant median difference of one bar was found. Mares (n = 4) demonstrated more faults and had a significantly greater difference in touches and faults between the two colours than male horses (n = 16). Repeating the same experiment with eight horses in an outdoor grass arena revealed no significant differences between the two colours. In order to draw any definite conclusions, more research concerning the colour perception, influence of contrast with the arena surface and sex of horse is required.