Gene duplication is an evolutionary mechanism for expanding spectral diversity in the long-wavelength photopigments of butterflies

Butterfly long-wavelength (L) photopigments are interesting for comparative studies of adaptive evolution because of the tremendous phenotypic variation that exists in their wavelength of peak absorbance (lambda(max) value). Here we present a comprehensive survey of L photopigment variation by measuring lambda(max) in 12 nymphalid and 1 riodinid species using epi-microspectrophotometry. Together with previous data, we find that L photopigment lambda(max) varies from 510-565 nm in 22 nymphalids, with an even broader 505- to 600-nm range in riodinids. We then surveyed the L opsin genes for which lambda(max) values are available as well as from related taxa and found 2 instances of L opsin gene duplication within nymphalids, in Hermeuptychia hermes and Amathusia phidippus, and 1 instance within riodinids, in the metalmark butterfly Apodemia mormo. Using maximum parsimony and maximum likelihood ancestral state reconstructions to map the evolution of spectral shifts within the L photopigments of nymphalids, we estimate the ancestral pigment had a lambda(max) = 540 nm +/- 10 nm standard error and that blueshifts in wavelength have occurred at least 4 times within the family. We used ancestral state reconstructions to investigate the importance of several amino acid substitutions (Ile17Met, Ala64Ser, Asn70Ser, and Ser137Ala) previously shown to have evolved under positive selection that are correlated with blue spectral shifts. These reconstructions suggest that the Ala64Ser substitution has indeed occurred along the newly identified blueshifted L photopigment lineages. Substitutions at the other 3 sites may also be involved in the functional diversification of L photopigments. Our data strongly suggest that there are limits to the evolution of L photopigment spectral shifts among species with only one L opsin gene and that opsin gene duplication broadens the potential range of lambda(max) values.     

The pineal organ as a folded retina: immunocytochemical localization of opsins

The most simple pineal complex (the pineal and parapineal organs of lampreys), consists of saccular evaginations of the diencephalic roof, and has a retina-like structure containing photoreceptor cells and secondary neurons. In more differentiated vertebrates, the successive folding of the pineal wall multiplies the cells and reduces the lumen of the organ, but the pattern of the histological organization remains similar to that of lampreys; therefore, we consider the histological structure of the pineal organ of higher vertebrates as a 'folded retina'. The cell membrane of several pineal photoreceptor outer-segments of vertebrates immunoreact with anti-retinal opsin antibodies supporting the view of retina-like organization of the pineal. Some other pineal outer segments do not react with retinal anti-opsin antibodies, a result suggesting the presence of special pineal photopigments in different types of pinealocytes that obviously developed during evolution. The chicken pinopsin, detected in the last years, may represent one of these unknown photopigments. Using antibodies against chicken pinopsin, we compared the immunoreactivity of different photoreceptors of the pineal organs from cyclostomes to birds at the light and electron microscopic levels. We found pinopsin immunoreaction on all pinealocytes of birds and on the rhodopsin-negative large reptilian pinealocytes. As the pinopsin has an absorption maximum at 470 nm, these avian and reptilian immunoreactive pinealocytes can be regarded as green-blue light-sensitive photoreceptors. Only a weak immunoreaction was observed on the frog and fish pinealocytes and no reaction was seen in cyclostomes and in the frontal organ of reptiles. Some photoreceptors of the retina of various species also reacted the pinopsin antibodies, therefore, pinopsin must have certain sequential similarity to individual retinal opsins of some vertebrates.     

Inner retinal photoreceptors (IRPs) in mammals and teleost fish.

Research over the past decade has provided overwhelming evidence that photoreception in the vertebrate eye is not confined to the rods and cones. The discovery of non-rod, non-cone ocular photoreceptors in mammals and fish arose from quite different lines of investigation. In transgenic mice entirely lacking functional rod and cone photoreceptors a range of responses to light, including the regulation of the circadian system and a pupillary light reflex, are preserved. Electrophysiological and imaging approaches were then able to characterise a coupled plexus of directly light sensitive ganglion cells. Most recently action spectroscopy has shown that a novel 'blue-light' sensitive photopigment based upon opsin/vitamin A (OP480) mediates these responses to light. Several candidate genes have emerged for OP480, with melanopsin being by far the strongest. A definitive link, however, between this gene and OP480 has still to be established. In contrast to the mammals, the discovery of inner retinal photoreceptors (IRPs) in fish started with the discovery of a new gene family (VA opsin). The teleost VA opsins form functional photopigments and are expressed in several different types of inner retinal neuron, including retinal horizontal cells. Recent studies have investigated the electrical properties of these photosensitive neurones, but their light-sensing role remains a matter of speculation. Thus the study of IRP is developing along quite separate lines. In the mammals the research is directed towards a molecular identification of the photopigment (OP480) and its cascade, whilst in fish the major effort is directed towards identifying a role for these novel photoreceptors using physiological approaches. The discovery of IRPs in the vertebrates tells us that despite 150 years of research, we still have much to learn about how the eye processes light.     

Expression of UV-, blue-, long-wavelength-sensitive opsins and melatonin in extraretinal photoreceptors of the optic lobes of hawkmoths

Lepidopterans display biological rhythms associated with egg laying, eclosion and flight activity but the photoreceptors that mediate these behavioural patterns are largely unknown. To further our progress in identifying candidate light-input channels for the lepidopteran circadian system, we have developed polyclonal antibodies against ultraviolet (UV)-, blue- and extraretinal long-wavelength (LW)-sensitive opsins and examined opsin immunoreactivity in the adult optic lobes of four hawkmoths, Manduca sexta, Acherontia atropos, Agrius convolvuli and Hippotion celerio. Outside the retina, UV and blue opsin protein expression is restricted to the adult stemmata, with no apparent expression elsewhere in the brain. Melatonin, which is known to have a seasonal influence on reproduction and behaviour, is expressed with opsins in adult stemmata together with visual arrestin and chaoptin. By contrast, the LW opsin protein is not expressed in the retina or stemmata but rather exhibits a distinct and widespread distribution in dorsal and ventral neurons of the optic lobes. The lamina, medulla, lobula and lobula plate, accessory medulla and adjacent neurons innervating this structure also exhibit strong LW opsin immunoreactivity. Together with the adult stemmata, these neurons appear to be functional photoreceptors, as visual arrestin, chaoptin and melatonin are also co-expressed with LW opsin. These findings are the first to suggest a role for three spectrally distinct classes of opsin in the extraretinal detection of changes in ambient light and to show melatonin-mediated neuroendocrine output in the entrainment of sphingid moth circadian and/or photoperiodic rhythms.This work was partially supported by the Canadian Institute for Advanced Research (A.D.B.) and the National Science Foundation (grant nos. IBN-0082700 and IBN-0346765; A.D.B.).     

Modeling the role of mid-wavelength cones in circadian responses to light

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.     

Magnetic Compass of Birds Is Based on a Molecule with Optimal Directional Sensitivity

The avian magnetic compass has been well characterized in behavioral tests: it is an “inclination compass” based on the inclination of the field lines rather than on the polarity, and its operation requires short-wavelength light. The “radical pair” model suggests that these properties reflect the use of specialized photopigments in the primary process of magnetoreception; it has recently been supported by experimental evidence indicating a role of magnetically sensitive radical-pair processes in the avian magnetic compass. In a multidisciplinary approach subjecting migratory birds to oscillating fields and using their orientation responses as a criterion for unhindered magnetoreception, we identify key features of the underlying receptor molecules. Our observation of resonance effects at specific frequencies, combined with new theoretical considerations and calculations, indicate that birds use a radical pair with special properties that is optimally designed as a receptor in a biological compass. This radical pair design might be realized by cryptochrome photoreceptors if paired with molecular oxygen as a reaction partner.     

Two isoforms of chicken melanopsins show blue light sensitivity

Melanopsin is a vertebrate non-visual opsin and functions as a circadian photoreceptor in mammalian retinas. Here we found the expression of two kinds of melanopsin genes in the chicken pineal gland and identified the presence of five isoforms derived from these two genes. Reconstitution of the recombinant proteins with 11-cis-retinal revealed that at least two of these melanopsin protein isoforms can function as blue-sensitive photopigments with absorption maxima at 476-484nm. These values are consistent with maximal sensitivities of action spectra determined from the physiological and behavioral studies on mammalian melanopsins. The melanopsin isoforms found in this study may function as pineal circadian photoreceptors.     

Opsin gene sequence variation across phylogenetic and population histories in Mysis (Crustacea: Mysida) does not match current light environments or visual-pigment absorbance spectra

The hypothesis that selection on the opsin gene is efficient in tuning vision to the ambient light environment of an organism was assessed in 49 populations of 12 Mysis crustacean species, inhabiting arctic marine waters, coastal littoral habitats, freshwater lakes ('glacial relicts') and the deep Caspian Sea. Extensive sequence variation was found within and among taxa, but its patterns did not match expectations based on light environments, spectral sensitivity of the visual pigment measured by microspectrophotometry or the history of species and populations. The main split in the opsin gene tree was between lineages I and II, differing in six amino acids. Lineage I was present in marine and Caspian Sea species and in the North American freshwater Mysis diluviana, whereas lineage II was found in the European and circumarctic fresh- and brackish-water Mysis relicta, Mysis salemaai and Mysis segerstralei. Both lineages were present in some populations of M. salemaai and M. segerstralei. Absorbance spectra of the visual pigment in nine populations of the latter three species showed a dichotomy between lake (λ(max) =554-562 nm) and brackish-water (Baltic Sea) populations (λ(max) = 521-535 nm). Judged by the shape of spectra, this difference was not because of different chromophores (A2 vs. A1), but neither did it coincide with the split in the opsin tree (lineages I/II), species identity or current light environments. In all, adaptive evolution of the opsin gene in Mysis could not be demonstrated, but its sequence variation did not conform to a neutral expectation either, suggesting evolutionary constraints and/or unidentified mechanisms of spectral tuning.     

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

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

Molecular ecology and adaptation of visual photopigments in craniates

In craniates, opsin‐based photopigments expressed in the eye encode molecular ‘light sensors’ that constitute the initial protein in photoreception and the activation of the phototransduction cascade. Since the cloning and sequencing of the first vertebrate opsin gene (bovine rod opsin) nearly 30 years ago (Ovchinnikov Yu 1982, FEBS Letters, 148, 179–191; Hargrave et al. 1983, Biophysics of Structure & Mechanism, 9, 235–244; Nathans & Hogness 1983, Cell, 34, 807–814), it is now well established that variation in the subtypes and spectral properties of the visual pigments that mediate colour and dim‐light vision is a prevalent mechanism for the molecular adaptation to diverse light environments. In this review, we discuss the origins and spectral tuning of photopigments that first arose in the agnathans to sample light within the ancient aquatic landscape of the Early Cambrian, detailing the molecular changes that subsequently occurred in each of the opsin classes independently within the main branches of extant jawed gnathostomes. Specifically, we discuss the adaptive changes that have occurred in the photoreceptors of craniates as they met the ecological challenges to survive in quite differing photic niches, including brightly lit aquatic surroundings; the deep sea; the transition to and from land; diurnal, crepuscular and nocturnal environments; and light‐restricted fossorial settings. The review ends with a discussion of the limitations inherent to the ‘nocturnal‐bottleneck’ hypothesis relevant to the evolution of the mammalian visual system and a proposition that transition through a ‘mesopic‐bottleneck’ may be a more appropriate model.     

Opportunism, photoperiodism, and puberty: Different mechanisms or variations on a theme?

There are many parallels between the neural regulation of seasonal breeding in birds and puberty in primates, but most studies of the regulation of puberty in vertebrates involve the use of rodents. The findings from rodent studies are often perceived as being typical of mammals and therefore pertinent to human reproductive biology and in many cases, rodent models have a great deal to offer in terms of an understanding of the regulation of puberty and reproductive biology. However, knowledge available from comparative work perhaps highlights mechanistic similarities that may not exist between rodent and primate systems. In this short review, we highlight some of the advantages of studying avian reproductive biology in this regard. We discuss disparities between rodent puberty and primate puberty, and similarities between primates and birds. Thus, understanding the mechanisms regulating avian puberty and seasonal breeding might in some cases provide greater insight into the mechanistic control of puberty in nonrodent mammals. We also describe in detail the neuroendocrine regulation of reproduction in birds and aim to provoke discussion of the possible roles of thyroid hormones and multiple forms of gonadotropin-releasing hormone in avian and mammalian reproduction.     

Switch in rod opsin gene expression in the European eel,Anguilla anguilla (L.).

The rod photoreceptors of the European eel, Anguilla anguilla (L.), alter their wavelength of maximum sensitivity (lambda max) from c.a. 523 nm to c.a. 482 nm at maturation, a switch involving the synthesis of a new visual pigment protein (opsin) that is inserted into the outer segments of existing rods. We artificially induced the switch in rod opsin production by the administration of hormones, and monitored the switch at the level of mRNA accumulation using radiolabelled oligonuleotides that hybridized differently to the two forms of eel rod opsin. The production of the deep-sea form of rod opsin was detected 6 h after the first hormone injection, and the switch in rod opsin expression was complete within four weeks, at which time only the mRNA for the deep-sea opsin was detectable in the retinal cells. It is suggested that this system could be used as a tractable model for studying the regulatory control of opsin gene expression.     

Photophysical diversity of two novel cyanobacteriochromes with phycocyanobilin chromophores: photochemistry and dark reversion kinetics

Cyanobacteriochromes are phytochrome homologues in cyanobacteria that act as sensory photoreceptors. We compare two cyanobacteriochromes, RGS (coded by slr1393) from Synechocystis sp. PCC 6803 and AphC (coded by all2699) from Nostoc sp. PCC 7120. Both contain three GAF (cGMP phosphodiesterase, adenylyl cyclase and FhlA protein) domains (GAF1, GAF2 and GAF3). The respective full-length, truncated and cysteine point-mutated genes were expressed in Escherichia coli together with genes for chromophore biosynthesis. The resulting chromoproteins were analyzed by UV-visible absorption, fluorescence and circular dichroism spectroscopy as well as by mass spectrometry. RGS shows a red-green photochromism (λ(max) = 650 and 535 nm) that is assigned to the reversible 15Z/E isomerization of a single phycocyanobilin-chromophore (PCB) binding to Cys528 of GAF3. Of the three GAF domains, only GAF3 binds a chromophore and the binding is autocatalytic. RGS autophosphorylates in vitro; this reaction is photoregulated: the 535 nm state containing E-PCB was more active than the 650 nm state containing Z-PCB. AphC from Nostoc could be chromophorylated at two GAF domains, namely GAF1 and GAF3. PCB-GAF1 is photochromic, with the proposed 15E state (λ(max) = 685 nm) reverting slowly thermally to the thermostable 15Z state (λ(max) = 635 nm). PCB-GAF3 showed a novel red-orange photochromism; the unstable state (putative 15E, λ(max) = 595 nm) reverts very rapidly (τ ~ 20 s) back to the thermostable Z state (λ(max) = 645 nm). The photochemistry of doubly chromophorylated AphC is accordingly complex, as is the autophosphorylation: E-GAF1/E-GAF3 shows the highest rate of autophosphorylation activity, while E-GAF1/Z-GAF3 has intermediate activity, and Z-GAF1/Z-GAF3 is the least active state.     

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

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

瘦素在禽类中的研究进展

瘦素(leptin)是一种主要作用于下丘脑的重要激素,起到调控摄食和能量消耗的作用。另外,国内外越来越多的研究表明,leptin与动物的代谢、发育、繁殖和免疫调节等均有密切的联系。但是上述研究大多在哺乳动物中进行,在禽类中的研究还在起步阶段。现有的研究表明,禽类的leptin及leptin受体的作用与哺乳动物相比都有其特殊性。本文首先分析了禽类leptin及leptin受体的特点,在此基础上,从摄食、生长发育和繁殖三个方面综述了leptin对禽类的作用及可能机制。     

Photoreceptors and photopigments in a subterranean rodent,the pocket gopher (<Emphasis Type="Italic">Thomomys bottae</Emphasis>)

Pocket gophers (Thomomys bottae) are rodents that spend much of their lives in near-lightless subterranean burrows. The visual adaptations associated with this extreme environment were investigated by making anatomical observations of retinal organization and by recording retinal responses to photic stimulation. The size of the eye is within the normal range for rodents, the lens transmits light well down into the ultraviolet, and the retina conforms to the normal mammalian plan. Electroretinogram recording revealed the presence of three types of photopigments, a rod pigment with a spectral peak of about 495 nm and two types of cone pigment with respective peak values of about 367 nm (UV) and 505 nm (medium-wavelength sensitive). Both in terms of responsivity to lights varying in temporal frequency and in response recovery following intense light adaptation, the cone responses of the pocket gopher are similar to those of other rodents. Labeling experiments indicate an abundance of cones that reach densities in excess of 30,000 mm^–2. Cones containing UV opsin are found throughout the retina, but those containing medium-wavelength sensitive opsin are mostly restricted to the dorsal retina where coexpression of the two photopigments is apparently the rule. Rod densities are lower than those typical for nocturnal mammals.     

Color Vision Variation as Evidenced by Hybrid L/M Opsin Genes in Wild Populations of Trichromatic Alouatta New World Monkeys

Platyrrhine (New World) monkeys possess highly polymorphic color vision owing to allelic variation of the single-locus L/M opsin gene on the X chromosome. Most species consist of female trichromats and female and male dichromats. Howlers (genus Alouatta) are an exception; they are considered to be routinely trichromatic with L and M opsin genes juxtaposed on the X chromosome, as seen in catarrhine primates (Old World monkeys, apes, and humans). Yet it is not known whether trichromacy is invariable in howlers. We examined L/M opsin variation in wild howler populations in Costa Rica and Nicaragua (Alouatta palliata) and Belize (A. pigra), using fecal DNA. We surveyed exon 5 sequences (containing the diagnostic 277th and 285th residues for λ_max) for 8 and 18 X chromosomes from Alouatta palliata and A. pigra, respectively. The wavelengths of maximal absorption (λ_max) of the reconstituted L and M opsin photopigments were 564 nm and 532 nm, respectively, in both species. We found one M–L hybrid sequence with a recombinant 277/285 haplotype in Alouatta palliata and two L–M hybrid sequences in A. pigra. The λ_max values of the reconstituted hybrid photopigments were in the range of 546~554 nm, which should result in trichromat phenotypes comparable to those found in other New World monkey species. Our finding of color vision variation due to high frequencies of L/M hybrid opsin genes in howlers challenges the current view that howlers are routine and uniform trichromats. These results deepen our understanding of the evolutionary significance of color vision polymorphisms and routine trichromacy and emphasize the need for further assessment of opsin gene variation as well as behavioral differences among subtypes of trichromacy.     

Activation of the blue opsin gene in cone photoreceptor development by retinoid-related orphan receptor beta

Color vision requires the expression of opsin photopigments with different wavelength sensitivities in retinal cone photoreceptors. The basic color visual system of mammals is dichromatic, involving differential expression in the cone population of two opsins with sensitivity to short (S, blue) or medium (M, green) wavelengths. However, little is known of the factors that directly activate these opsin genes and thereby contribute to the S or M opsin identity of the cone. We report that the orphan nuclear receptor RORbeta (retinoid-related orphan receptor beta) activates the S opsin gene (Opn1sw) through binding sites upstream of the gene. RORbeta lacks a known physiological ligand and activates the Opn1sw promoter modestly alone but strongly in synergy with the retinal cone-rod homeobox factor (CRX), suggesting a cooperative means of enhancing RORbeta activity. Comparison of wild-type and mutant lacZ reporter transgenes showed that the RORbeta-binding sites in Opn1sw are required for expression in mouse retina. RORbeta-deficient mice fail to induce S opsin appropriately during postnatal cone development. Photoreceptors in these mice also lack outer segments, indicating additional functions for RORbeta in photoreceptor morphological maturation. The results identify Opn1sw as a target gene for RORbeta and suggest a key role for RORbeta in regulating opsin expression in the color visual system.     

Photoreceptor spectral sensitivities in terrestrial animals: adaptations for luminance and colour vision

This review outlines how eyes of terrestrial vertebrates and insects meet the competing requirements of coding both spatial and spectral information. There is no unique solution to this problem. Thus, mammals and honeybees use their long-wavelength receptors for both achromatic (luminance) and colour vision, whereas flies and birds probably use separate sets of photoreceptors for the two purposes. In particular, we look at spectral tuning and diversification among 'long-wavelength' receptors (sensitivity maxima at greater than 500 nm), which play a primary role in luminance vision. Data on spectral sensitivities and phylogeny of visual photopigments can be incorporated into theoretical models to suggest how eyes are adapted to coding natural stimuli. Models indicate, for example, that animal colour vision--involving five or fewer broadly tuned receptors--is well matched to most natural spectra. We can also predict that the particular objects of interest and signal-to-noise ratios will affect the optimal eye design. Nonetheless, it remains difficult to account for the adaptive significance of features such as co-expression of photopigments in single receptors, variation in spectral sensitivities of mammalian L-cone pigments and the diversification of long-wavelength receptors that has occurred in several terrestrial lineages.