Long-wavelength-sensitive Research Articles

Red vision in animals is broadly associated with lighting environment but not types of visual task.

Red sensitivity is the exception rather than the norm in most animal groups. Among species with red sensitivity, there is substantial variation in the peak wavelength sensitivity (λmax) of the long wavelength sensitive (LWS) photoreceptor. It is unclear whether this variation can be explained by visual tuning to the light environment or to visual tasks such as signalling or foraging. Here, we examine long wavelength sensitivity across a broad range of taxa showing diversity in LWS photoreceptor λmax: insects, crustaceans, arachnids, amphibians, reptiles, fish, sharks and rays. We collated a list of 161 species with physiological evidence for a photoreceptor sensitive to red wavelengths (i.e. λmax ≥ 550 nm) and for each species documented abiotic and biotic factors that may be associated with peak sensitivity of the LWS photoreceptor. We found evidence supporting visual tuning to the light environment: terrestrial species had longer λmax than aquatic species, and of these, species from turbid shallow waters had longer λmax than those from clear or deep waters. Of the terrestrial species, diurnal species had longer λmax than nocturnal species, but we did not detect any differences across terrestrial habitats (closed, intermediate or open). We found no association with proxies for visual tasks such as having red morphological features or utilising flowers or coral reefs. These results support the emerging consensus that, in general, visual systems are broadly adapted to the lighting environment and diverse visual tasks. Links between visual systems and specific visual tasks are commonly reported, but these likely vary among species and do not lead to general patterns across species.

Knockout of SWS2 in zebrafish (Danio rerio) reveals its roles in feeding and phototactic behaviors

Common ancestor of vertebrates had four cone opsin subfamilies to obtain color vision: ultraviolet-sensitive (SWS1), blue-sensitive (SWS2), middle wavelength sensitive (RH2) and long wavelength sensitive (LWS). Nevertheless, eutherian mammals had lost the SWS2 and RH2 opsins during their nocturnal lifestyle. Many studies had demonstrated the role of SWS1 and LWS cones in feeding, mate choice and skin pigment cell formation. However, the role of SWS2 and RH2 cones remain elusive. In the present study, we used an ideal model visual system, zebrafish, which still have the four cone opsins, to generate a SWS2 knockout zebrafish line. Through various behavioral test, we found that sws2-/- zebrafish larvae exhibited increased food intake compared with WT. Additionally, there were significantly increased the gene expression of phototransduction pathways in sws2-/- zebrafish larvae. Compared to WT, mutant zebrafish showed weaker phototaxis of red light and changed sensitivity of yellow, red and blue lights. But both mutant and WT zebrafish preferred the red light than other wavelengths of light. Taken together, we proposed that SWS2 cone is not necessary for feeding and phototaxis in zebrafish.

Opsin Gene Duplication in Lepidoptera: Retrotransposition, Sex Linkage, and Gene Expression.

Color vision in insects is determined by signaling cascades, central to which are opsin proteins, resulting in sensitivity to light at different wavelengths. In certain insect groups, lineage-specific evolution of opsin genes, in terms of copy number, shifts in expression patterns, and functional amino acid substitutions, has resulted in changes in color vision with subsequent behavioral and niche adaptations. Lepidoptera are a fascinating model to address whether evolutionary change in opsin content and sequence evolution are associated with changes in vision phenotype. Until recently, the lack of high-quality genome data representing broad sampling across the lepidopteran phylogeny has greatly limited our ability to accurately address this question. Here, we annotate opsin genes in 219 lepidopteran genomes representing 33 families, reconstruct their evolutionary history, and analyze shifts in selective pressures and expression between genes and species. We discover 44 duplication events in opsin genes across ∼300 million years of lepidopteran evolution. While many duplication events are species or family specific, we find retention of an ancient long-wavelength-sensitive (LW) opsin duplication derived by retrotransposition within the speciose superfamily Noctuoidea (in the families Nolidae, Erebidae, and Noctuidae). This conserved LW retrogene shows life stage-specific expression suggesting visual sensitivities or other sensory functions specific to the early larval stage. This study provides a comprehensive order-wide view of opsin evolution across Lepidoptera, showcasing high rates of opsin duplications and changes in expression patterns.

Role of short-wave-sensitive 1 (sws1) in cone development and first feeding in larval zebrafish.

Color vision is mediated by the expression of different major visual pigment proteins (opsins) on retinal photoreceptors. Vertebrates have four classes of cone opsins that are most sensitive to different wavelengths of light: short wavelength sensitive 1 (SWS1), short wavelength sensitive 2 (SWS2), medium wavelength sensitive (RH2), and long wavelength sensitive (LWS). UV wavelengths play important roles in foraging and communication. However, direct evidence provide links between sws1 and first feeding is lacking. Here, CRISPR/Cas9 technology was performed to generate mutant zebrafish lines with sws1 deletion. sws1 mutant zebrafish larvae exhibited decreased sws1, rh2-2, and lws1 expression, and increased rod gene (rho and gnat1) expression. Furthermore, the sws1-deficient larvae exhibited significantly reduced food intake, and the orexigenic genes npy and agrp signaling were upregulated at 6days postfertilization (dpf). The transcription expression of sws1 and rh2-3 genes decreased in sws1-/- adults compared to wild type. Surprisingly, the results of feeding at the adult stage were not the same with larvae. sws1 deficiency did not affect food intake and appetite gene expression at adult stages. These results reveal a role for sws1 in normal cone development and first feeding in larval zebrafish.

Expression and function of opsin genes associated with phototaxis in Zeugodacus cucurbitae Coquillett (Diptera: Tephritidae).

Zeugodacus cucuribitae is a major agricultural pest that causes significant damage to varieties of plants. Vision plays a critical role in phototactic behavior of herbivorous insects. However, the effect of opsin on the phototactic behavior in Z.cucuribitae remain unknown. The aim of the research is to explore the key opsin genes that associate with phototaxis behavior of Z. cucurbitae. We identified five opsin genes and analyzed their expression patterns. The relative expression levels of ZcRh1, ZcRh4 and ZcRh6 were highest in 4-day-old larvae, ZcRh2 and ZcRh3 were highest in 3rd-instar larvae and 5-day-old pupae, respectively. Furthermore, five opsin genes had the highest expression levels in compound eyes, followed by the antennae and head, whereas the lower occurred in other tissues. The expression of the long-wavelength-sensitive (LW) opsins first decreased and then increased under green light exposure. In contrast, the expression of ultraviolet-sensitive (UV) opsins first increased and then decreased with the duration of UV exposure. Silencing of LW opsin (dsZcRh1, dsZcRh2, and dsZcRh6) and UV opsin (dsZcRh3 and dsZcRh4) reduced the phototactic efficiency of Z. cucurbitae to green light by 52.27%, 60.72%, and 67.89%, and to UV light by 68.59% and 61.73%, respectively. The study found that RNAi inhibited the expression of opsin, thereby inhibiting the phototaxis of Z. cucurbitae. This result provides theoretical support for the physical control of Z. cucurbitae and lays the foundation for further exploration of the mechanism of insect phototaxis. This article is protected by copyright. All rights reserved.

The chromosome-level genome of Chinese praying mantis Tenodera sinensis (Mantodea: Mantidae) reveals its biology as a predator.

The Chinese praying mantis, Tenodera sinensis (Saussure), is a carnivorous insect that preys on a variety of arthropods and small vertebrates, including pest species. Several studies have been conducted to understand its behavior and physiology. However, there is limited knowledge about the genetic information underlying its genome evolution, digestive demands, and predatory behaviors. Here we have assembled the chromosome-level genome of T. sinensis, representing the first sequenced genome of the family Mantidae, with a genome size of 2.54 Gb and scaffold N50 of 174.78 Mb. Our analyses revealed that 98.6% of BUSCO genes are present, resulting in a well-annotated assembly compared to other insect genomes, containing 25,022 genes. The reconstructed phylogenetic analysis showed the expected topology placing the praying mantis in an appropriate position. Analysis of transposon elements suggested the Gypsy/Dirs family, which belongs to long terminal repeat (LTR) transposons, may be a key factor resulting in the larger genome size. The genome shows expansions in several digestion and detoxification associated gene families, including trypsin and glycosyl hydrolase (GH) genes, ATP-binding cassette (ABC) transporter, and carboxylesterase (CarE), reflecting the possible genomic basis of digestive demands. Furthermore, we have found 1 ultraviolet-sensitive opsin and 2 long-wavelength-sensitive (LWS) opsins, emphasizing the core role of LWS opsins in regulating predatory behaviors. The high-quality genome assembly of the praying mantis provides a valuable repository for studying the evolutionary patterns of the mantis genomes and the gene expression profiles of insect predators.

Crustacean conundrums: a review of opsin diversity and evolution.

Knowledge of crustacean vision is lacking compared to the more well-studied vertebrates and insects. While crustacean visual systems are typically conserved morphologically, the molecular components (i.e. opsins) remain understudied. This review aims to characterize opsin diversity across crustacean lineages for an integrated view of visual system evolution. Using publicly available data from 95 species, we identified opsin sequences and classified them by clade. Our analysis produced 485 putative visual opsins and 141 non-visual opsins. The visual opsins were separated into six clades: long wavelength sensitive (LWS), middle wavelength sensitive (MWS) 1 and 2, short wavelength or ultraviolet sensitive (SWS/UVS) and a clade of thecostracan opsins, with multiple LWS and MWS opsin copies observed. The SWS/UVS opsins were relatively conserved in most species. The crustacean classes Cephalocarida, Remipedia and Hexanauplia exhibited reduced visual opsin diversity compared to others, with the malacostracan decapods having the highest opsin diversity. Non-visual opsins were identified from all investigated classes except Cephalocarida. Additionally, a novel clade of non-visual crustacean-specific, R-type opsins (Rc) was discovered. This review aims to provide a framework for future research on crustacean vision, with an emphasis on the need for more work in spectral characterization and molecular analysis. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Correlated Expression of the Opsin Retrogene LWS-R and its Host Gene in Two Poeciliid Fishes.

The important role of retrogenes in genome evolution and species differentiation is becoming increasingly accepted. One synapomorphy among cyprinodontoid fish is a retrotransposed version of a long-wavelength sensitive (LWS) opsin gene, LWS-R, within an intron of the gephyrin (GPHN) gene. These two genes display opposing orientations. It had been speculated that LWS-R hijacks the cis-regulatory elements of GPHN for transcription, but whether their expression is correlated had remained unclear. Here, in silico predictions identified putative promoters upstream of the translation start site of LWS-R, indicating that its transcription is driven by its own promoter rather than by the GPHN promoter. However, consistent expression ratios of LWS-R:GPHN in the eyeball and brain of fishes indicate that the respective gene transcriptions are correlated. Co-expression is potentially modulated by histone exchange during GPHN transcription. Two isoforms were detected in this study, i.e., intron-free and intron-retaining. Intron-free LWS-R was only expressed in the eyeball of fishes, whereas intron-retaining LWS-R occurred in both eyeball and brain. Expression of vision-associated LWS-R beyond the eyeball supports that it is co-expressed with more ubiquitous GPHN.

Amber light treatment produces hyperopia in tree shrews.

Exposure to narrow-band red light, which stimulates only the long-wavelength sensitive (LWS) cones, slows axial eye growth and produces hyperopia in tree shrews and macaque monkeys. We asked whether exposure to amber light, which also stimulates only the LWS cones but with a greater effective illuminance than red light, has a similar hyperopia-inducing effect in tree shrews. Starting at 24±1days of visual experience, 15 tree shrews (dichromatic mammals closely related to primates) received light treatment through amber filters (BPI 500/550 dyed acrylic) either atop the cage (Filter group, n=8, 300-400human lux) or fitted into goggles in front of both eyes (Goggle group, n=7). Non-cycloplegic refractive error and axial ocular dimensions were measured daily. Treatment groups were compared with age-matched animals (Colony group, n=7) raised in standard colony fluorescent lighting (100-300lux). At the start of treatment, mean refractive errors were well-matched across the three groups (p=0.35). During treatment, the Filter group became progressively more hyperopic with age (p<0.001). By contrast, the Goggleand Colony groups showed continued normal emmetropization. When the treatment ended, the Filtergroup exhibited significantly greater hyperopia (mean [SE]=3.5 [0.6] D) compared with the Goggle (0.2[0.8]D, p=0.01) and Colony groups (1.0 [0.2] D, p=0.01). However, the refractive error in the Goggle group was not different from that in the Colony group (p=0.35). Changes in the vitreous chamber were consistent with the refractive error changes. Exposure to ambient amber light produced substantial hyperopia in the Filter group but had no effect on refractive error in the Goggle group. The lack of effect in the Goggle group could be due to the simultaneous activation of the short-wavelength sensitive (SWS) and LWS cones caused by the scattering of the broad-band light from the periphery of the goggles.

Behavioural red-light sensitivity in fish according to the optomotor response.

Various procedures have been adopted to investigate spectral sensitivity of animals, e.g. absorption spectra of visual pigments, electroretinography, optokinetic response, optomotor response (OMR) and phototaxis. The use of these techniques has led to various conclusions about animal vision. However, visual sensitivity should be evaluated consistently for a reliable comparison. In this study, we retrieved behavioural data of several fish species using a single OMR procedure and compared their sensitivities to near-infrared light. Besides cavefish that lack eyes, some species were not appropriate for the OMR test because they either stayed still or changed swimming direction frequently. Eight of 13 fish species tested were OMR positive. Detailed analyses using medaka, goldfish, zebrafish, guppy, stickleback and cichlid revealed that all the fish were sensitive to light at a wavelength greater than or equal to 750 nm, where the threshold wavelengths varied from 750 to 880 nm. Fish opsin repertoire affected the perception of red light. By contrast, the copy number of long-wavelength-sensitive (LWS) genes did not necessarily improve red-light sensitivity. While the duplication of LWS and other cone opsin genes that has occurred extensively during fish evolution might not aid increasing spectral sensitivity, it may provide some other advantageous ophthalmic function, such as enhanced spectral discrimination.

Chimeric human opsins as optogenetic light sensitisers.

ABSTRACTHuman opsin-based photopigments have great potential as light-sensitisers, but their requirement for phototransduction cascade-specific second messenger proteins may restrict their functionality in non-native cell types. In this study, eight chimeric human opsins were generated consisting of a backbone of either a rhodopsin (RHO) or long-wavelength-sensitive (LWS) opsin and intracellular domains from Gq/11-coupled human melanopsin. Rhodopsin/melanopsin chimeric opsins coupled to both Gi and Gq/11 pathways. Greater substitution of the intracellular surface with corresponding melanopsin domains generally showed greater Gq/11 activity with a decrease in Gi activation. Unlike melanopsin, rhodopsin and rhodopsin/melanopsin chimeras were dependent upon exogenous chromophore to function. By contrast, wild-type LWS opsin and LWS opsin/melanopsin chimeras showed only weak Gi activation in response to light, whilst Gq/11 pathway activation was not detected. Immunocytochemistry (ICC) demonstrated that chimeric opsins with more intracellular domains of melanopsin were less likely to be trafficked to the plasma membrane. This study demonstrates the importance of Gα coupling efficiency to the speed of cellular responses and created human opsins with a unique combination of properties to expand the range of customised optogenetic biotools for basic research and translational therapies.

Draft Genome of the Mirrorwing Flyingfish (Hirundichthys speculiger).

Draft Genome of the Mirrorwing Flyingfish (Hirundichthys speculiger).

The diversity of opsins in Lake Baikal amphipods (Amphipoda: Gammaridae)

BackgroundVision is a crucial sense for the evolutionary success of many animal groups. Here we explore the diversity of visual pigments (opsins) in the transcriptomes of amphipods (Crustacea: Amphipoda) and conclude that it is restricted to middle (MWS) and long wavelength-sensitive (LWS) opsins in the overwhelming majority of examined species.ResultsWe evidenced (i) parallel loss of MWS opsin expression in multiple species (including two independently evolved lineages from the deep and ancient Lake Baikal) and (ii) LWS opsin amplification (up to five transcripts) in both Baikal lineages. The number of LWS opsins negatively correlated with habitat depth in Baikal amphipods. Some LWS opsins in Baikal amphipods contained MWS-like substitutions, suggesting that they might have undergone spectral tuning.ConclusionsThis repeating two-step evolutionary scenario suggests common triggers, possibly the lack of light during the periods when Baikal was permanently covered with thick ice and its subsequent melting. Overall, this observation demonstrates the possibility of revealing climate history by following the evolutionary changes in protein families.

Evolutionary conservation of opsin gene expression patterns in the compound eyes of darkling beetles.

Recent large-scale studies of opsin gene contents in representatives of the largest order of insects, the Coleoptera (beetles), revealed that the blue wavelength-sensitive (B) opsin subfamily is absent in this clade, while the ultraviolet- (UV) and long wavelength-sensitive (LW) opsin subfamilies are broadly conserved with gene duplications possibly reintroducing blue sensitivity in select subclades. Little is known yet, however, how opsin genes are expressed in the compound eyes of beetles. In a previous study, we analyzed opsin gene expression in the red flour beetle Tribolium castaneum, a member of the family of darkling beetles (Tenebrionidae), and found that a singleton LW opsin homolog is homogeneously expressed in all photoreceptors of the compound eye retina with a singleton UV opsin homolog being co-expressed in the R7 subtype photoreceptors. To probe for the evolutionary conservation of these expression patterns, we isolated complete opsin transcript sequences from three additional species in the subfamily Tenebrionidae (Tribolium confusum, Tenebrio molitor, Zophobas morio) and studied their expression via whole mount in situ hybridization in the pupal retina. These experiments revealed very similar, if not identical, photoreceptor subtype-specific expression patterns in all three species compared with T. castaneum. Documenting a deep conservation of photoreceptor subtype-specific opsin gene expression in this range of darkling beetles, our study provides a first point of reference for broader comparative studies of retinal organization in the Coleoptera.

Changes in a Cone Opsin Repertoire Affect Color-Dependent Social Behavior in Medaka but Not Behavioral Photosensitivity.

Common ancestors of vertebrates had four types of cone opsins: short-wavelength sensitive 1 (SWS1), SWS2, rhodopsin 2 (RH2), and long-wavelength sensitive (LWS) types. Whereas fish and birds retain all the types, mammals have lost two of them (SWS2 and RH2) possibly because of their nocturnal lifestyle during the Mesozoic Era. Considering that the loss of cone opsin types causes so-called color blindness in humans (e.g., protanopia), the ability to discriminate color by trichromatic humans could be lower than that in potentially tetrachromatic birds and fish. Behavioral studies using color-blind (cone opsin-knockout) animals would be helpful to address such questions, but it is only recently that the genome-editing technologies have opened up this pathway. Using medaka as a model, we introduced frameshift mutations in SWS2 (SWS2a and/or SWS2b) after detailed characterization of the loci in silico, which unveiled the existence of a GC–AG intron and non-optic expressed-sequence-tags (ESTs) that include SWS2a in part. Transcripts from the mutated SWS2 loci are commonly reduced, suggesting that the SWS2a/b-double mutants could produce, if any, severely truncated (likely dysfunctional) SWS2s in small amounts. The mutants exhibited weakened body color preferences during mate choice. However, the optomotor response (OMR) test under monochromatic light revealed that the mutants had no defect in spectral sensitivity, even at the absorbance maxima (λmax) of SWS2s. Evolutionary diversification of cone opsins has often been discussed in relation to adaptation to dominating light in habitats (i.e., changes in the repertoire or λmax are for increasing sensitivity to the dominating light). However, the present results seem to provide empirical evidence showing that acquiring or losing a type of cone opsin (or changes in λmax) need not substantially affect photopic or mesopic sensitivity. Other points of view, such as color discrimination of species-specific mates/preys/predators against habitat-specific backgrounds, may be necessary to understand why cone opsin repertories are so various among animals.

An opponent dual-detector spectral drive model of emmetropization

In post-natal developing eyes a feedback mechanism uses optical cues to regulate axial growth so as to achieve good focus, a process termed emmetropization. However, the optical cues that the feedback mechanism uses have remained unclear. Here we present evidence that a primary visual cue may be the detection of different image statistics by the short-wavelength sensitive (SWS) and long-wavelength sensitive (LWS) cone photoreceptors, caused by longitudinal chromatic aberration (LCA). We use as a model system the northern tree shrew Tupaia belangeri, diurnal cone-dominated dichromatic mammals closely related to primates. We present an optical model in which the SWS and LWS photoreceptors each represent an image at different levels of defocus. The model posits that an imbalance between SWS and LWS image statistics directs eye growth towards the point at which these image statistics are in balance. Under spectrally broadband (“white”) lighting, the focus of the eye is driven to a target point approximately in the middle of the visible spectrum, which is emmetropia. Calculations suggest that the SWS cone array, despite the sparse number of SWS cones, can plausibly detect the wavelength-dependent differences in defocus and guide refractive development. The model is consistent with the effects of various narrow-band illuminants on emmetropization in tree shrews. Simulations suggest that common artificial light spectra do not interfere with emmetropization. Simulations also suggest that multi-spectral multi-focal lenses, where the different optical zones of a multifocal lens have different spectral filtering properties, could be an anti-myopia intervention.

Tbx2a Modulates Switching of RH2 and LWS Opsin Gene Expression.

Sensory systems are tuned by selection to maximize organismal fitness in particular environments. This tuning has implications for intraspecies communication, the maintenance of species boundaries, and speciation. Tuning of color vision largely depends on the sequence of the expressed opsin proteins. To improve tuning of visual sensitivities to shifts in habitat or foraging ecology over the course of development, many organisms change which opsins are expressed. Changes in this developmental sequence (heterochronic shifts) can create differences in visual sensitivity among closely related species. The genetic mechanisms by which these developmental shifts occur are poorly understood. Here, we use quantitative trait locus analyses, genome sequencing, and gene expression studies in African cichlid fishes to identify a role for the transcription factor Tbx2a in driving a switch between long wavelength sensitive (LWS) and Rhodopsin-like (RH2) opsin expression. We identify binding sites for Tbx2a in the LWS promoter and the highly conserved locus control region of RH2 which concurrently promote LWS expression while repressing RH2 expression. We also present evidence that a single change in Tbx2a regulatory sequence has led to a species difference in visual tuning, providing the first mechanistic model for the evolution of rapid switches in sensory tuning. This difference in visual tuning likely has important roles in evolution as it corresponds to differences in diet, microhabitat choice, and male nuptial coloration.

Spectral tuning mediated by helix III in butterfly long wavelength-sensitive visual opsins revealed by heterologous action spectroscopy

Absorption spectra of opsin-based pigments are tuned from the UV to the red regions by interactions of the chromophore with surrounding amino acid residues. Both vertebrates and invertebrates possess long-wavelength-sensitive (LWS) opsins, which underlie color vision involving “red” sensing. The LWS opsins have independently evolved in each lineage, which suggests the existence of diverse mechanisms in spectral tuning. In vertebrate LWS opsins, the mechanisms underlying spectral tuning have been well characterized by spectroscopic analyses with recombinant pigments of wild type (WT) and mutant opsins. However in invertebrate LWS opsins including insect ones, the mechanisms are largely unknown due to the difficulty in obtaining recombinant pigments. Here we have overcome the problem by analyzing heterologous action spectra based on light-dependent changes in the second messenger in opsin-expressing cultured cells. We found that WTs of two LWS opsins of the butterfly, Papilio xuthus, PxRh3 and PxRh1 have the wavelengths of the absorption maxima at around 570 nm and 540 nm, respectively. Analysis of a series of chimeric mutants showed that helix III is crucial to generating a difference of about 15 nm in the wavelength of absorption maxima of these LWS opsins. Further site-directed mutations in helix III revealed that amino acid residues at position 116 and 120 (bovine rhodopsin numbering system) are involved in the spectral tuning of PxRh1 and PxRh3, suggesting a different spectral tuning mechanism from that of primate LWS opsins.

LWS visual pigment in owls: Spectral tuning inferred by genetics

Owls constitute a diverse group of raptors, active at different times of the day with distinct light conditions that might be associated with multiple visual adaptations. We investigated whether shifts in the spectral sensitivity of the L cone visual pigment, as inferred by analysis of gene structure, could be one such adaptive mechanism. Using Sanger sequencing approach, we characterized the long wavelength-sensitive (LWS) opsin gene expressed in the retina of five owl species, specifically chosen to represent distinct patterns of activity. Nocturnality was epitomized by the American barn owl (Tyto furcata), the striped owl (Asio clamator), and the tropical screech owl (Megascops choliba); diurnality, by the ferruginous pygmy owl (Glaucudium brasilianum); and cathemerality, by the burrowing owl (Athene cunicularia). We also analyzed the presence of the L cone in the retinas of four species of owl (T. furcata, A. cunicularia, G. brasilianum and M. choliba) using immnunohistochemistry. Five critical sites for the spectral tuning of the LWS opsin (164, 181, 261, 269, and 292) were analyzed and compared to the sequence of other birds. The sequence of A. cunicularia showed a substitution on residue 269, with the presence of an alanine instead threonine, which generates an estimated maximum absorption (λmax) around 537 nm. No other variation was found in the spectral tuning sites of the LWS opsin, among the other species, and the λmax was estimated at around 555 nm. The presence of L cones in the retinas of the four species of owls was revealed using immunohistochemistry and we observed a reduced number of L cones in T. furcata compared to A. cunicularia, G. brasilianum and M. choliba.

The retinal pigments of the whale shark (Rhincodon typus) and their role in visual foraging ecology.

The spectral tuning properties of the whale shark (Rhincodon typus) rod (rhodopsin or Rh1) and long-wavelength-sensitive (LWS) cone visual pigments were examined to determine whether these retinal pigments have adapted to the broadband light spectrum available for surface foraging or to the narrowband blue-shifted light spectrum available at depth. Recently published whale shark genomes have identified orthologous genes for both the whale shark Rh1 and LWS cone opsins suggesting a duplex retina. Here, the whale shark Rh1 and LWS cone opsin sequences were examined to identify amino acid residues critical for spectral tuning. Surprisingly, the predicted absorbance maximum (λmax) for both the whale shark Rh1 and LWS visual pigments is near 500 nm. Although Rh1 λmax values near 500 nm are typical of terrestrial vertebrates, as well as surface foraging fish, it is uncommon for a vertebrate LWS cone pigment to be so greatly blue-shifted. We propose that the spectral tuning properties of both the whale shark Rh1 and LWS cone pigments are most likely adaptations to the broadband light spectrum available at the surface. Whale shark melanopsin (Opn4) deactivation kinetics was examined to better understand the underlying molecular mechanisms of the pupillary light reflex. Results show that the deactivation rate of whale shark Opn4 is similar to the Opn4 deactivation rate from vertebrates possessing duplex retinae and is significantly faster than the Opn4 deactivation rate from an aquatic rod monochromat lacking functional cone photoreceptors. The rapid deactivation rate of whale shark Opn4 is consistent with a functional cone class and would provide the animal with an exponential increase in the number of photons required for photoreceptor signaling when transitioning from photopic to scotopic light conditions, as is the case when diving.