Spectral tuning of a circadian photopigment in a subterranean 'blind' mammal (Spalax ehrenbergi)

Alone, in the dark: The extraordinary neuroethology of the solitary blind mole rat

On the social scale, the blind mole rat (BMR; Spalax ehrenbergi) is an extreme. It is exceedingly solitary, territorial, and aggressive. BMRs reside underground, in self-excavated tunnels that they rarely leave. They possess specialized sensory systems for social communication and navigation, which allow them to cope with the harsh environmental conditions underground. This review aims to present the blind mole rat as an ideal, novel neuroethological model for studying aggressive and solitary behaviors. We discuss the BMR's unique behavioral phenotype, particularly in the context of 'anti-social' behaviors, and review the available literature regarding its specialized sensory adaptations to the social and physical habitat. To date, the neurobiology of the blind mole rat remains mostly unknown and holds a promising avenue for scientific discovery. Unraveling the neural basis of the BMR's behavior, in comparison to that of social rodents, can shed important light on the underlying mechanisms of psychiatric disorders in humans, in which similar behaviors are displayed.

A natural history of vertebrate vision loss: Insight from mammalian vision for human visual function

Research on the origin of vision and vision loss in naturally "blind" animal species can reveal the tasks that vision fulfills and the brain's role in visual experience. Models that incorporate evolutionary history, natural variation in visual ability, and experimental manipulations can help disentangle visual ability at a superficial level from behaviors linked to vision but not solely reliant upon it, and could assist the translation of ophthalmological research in animal models to human treatments. To unravel the similarities between blind individuals and blind species, we review concepts of 'blindness' and its behavioral correlates across a range of species. We explore the ancestral emergence of vision in vertebrates, and the loss of vision in blind species with reference to an evolution-based classification scheme. We applied phylogenetic comparative methods to a mammalian tree to explore the evolution of visual acuity using ancestral state estimations. Future research into the natural history of vision loss could help elucidate the function of vision and inspire innovations in how to address vision loss in humans.

Evolutionary Constraint on Visual and Nonvisual Mammalian Opsins

Animals have evolved light-sensitive G protein-coupled receptors, known as opsins, to detect coherent and ambient light for visual and nonvisual functions. These opsins have evolved to satisfy the particular lighting niches of the organisms that express them. While many unique patterns of evolution have been identified in mammals for rod and cone opsins, far less is known about the atypical mammalian opsins. Using genomic data from over 400 mammalian species from 22 orders, unique patterns of evolution for each mammalian opsins were identified, including photoisomerases, RGR-opsin (RGR) and peropsin (RRH), as well as atypical opsins, encephalopsin (OPN3), melanopsin (OPN4), and neuropsin (OPN5). The results demonstrate that OPN5 and rhodopsin show extreme conservation across all mammalian lineages. The cone opsins, SWS1 and LWS, and the nonvisual opsins, OPN3 and RRH, demonstrate a moderate degree of sequence conservation relative to other opsins, with some instances of lineage-specific gene loss. Finally, the photoisomerase, RGR, and the best-studied atypical opsin, OPN4, have high sequence diversity within mammals. These conservation patterns are maintained in human populations. Importantly, all mammalian opsins retain key amino acid residues important for conjugation to retinal-based chromophores, permitting light sensitivity. These patterns of evolution are discussed along with known functions of each atypical opsin, such as in circadian or metabolic physiology, to provide insight into the observed patterns of evolutionary constraint.

Circadian Photoentrainment in Mice and Humans

Light around twilight provides the primary entrainment signal for circadian rhythms. Here we review the mechanisms and responses of the mouse and human circadian systems to light. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λ_max close to 480 nm. Anatomical findings demonstrate that there are multiple pRGC sub-types, with some evidence in mice, but little in humans, regarding their roles in regulating physiology and behavior. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. Such an integration of signals enables the rods to detect dim light, the cones to detect higher light intensities and the integration of intermittent light exposure, whilst melanopsin measures bright light over extended periods of time. Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. Mice can entrain to light at approximately 1 lux for a few minutes, whilst humans require light at high irradiance (>100’s lux) and of a long duration (>30 min). The basis for this difference remains unclear. As our retinal light exposure is highly dynamic, and because photoreceptor interactions are complex and difficult to model, attempts to develop evidence-based lighting to enhance human circadian entrainment are very challenging. A way forward will be to define human circadian responses to artificial and natural light in the “real world” where light intensity, duration, spectral quality, time of day, light history and age can each be assessed.

Non-image Forming Light Detection by Melanopsin, Rhodopsin, and Long-Middlewave (L/W) Cone Opsin in the Subterranean Blind Mole Rat, Spalax Ehrenbergi: Immunohistochemical Characterization, Distribution, and Connectivity

The blind mole rat, Spalax ehrenbergi, can, despite severely degenerated eyes covered by fur, entrain to the daily light/dark cycle and adapt to seasonal changes due to an intact circadian timing system. The present study demonstrates that the Spalax retina contains a photoreceptor layer, an outer nuclear layer (ONL), an outer plexiform layer (OPL), an inner nuclear layer (INL), an inner plexiform layer (IPL), and a ganglion cell layer (GCL). By immunohistochemistry, the number of melanopsin (mRGCs) and non-melanopsin bearing retinal ganglion cells was analyzed in detail. Using the ganglion cell marker RNA-binding protein with multiple splicing (RBPMS) it was shown that the Spalax eye contains 890 ± 62 RGCs. Of these, 87% (752 ± 40) contain melanopsin (cell density 788 melanopsin RGCs/mm²). The remaining RGCs were shown to co-store Brn3a and calretinin. The melanopsin cells were located mainly in the GCL with projections forming two dendritic plexuses located in the inner part of the IPL and in the OPL. Few melanopsin dendrites were also found in the ONL. The Spalax retina is rich in rhodopsin and long/middle wave (L/M) cone opsin bearing photoreceptor cells. By using Ctbp2 as a marker for ribbon synapses, both rods and L/M cone ribbons containing pedicles in the OPL were found in close apposition with melanopsin dendrites in the outer plexus suggesting direct synaptic contact. A subset of cone bipolar cells and all photoreceptor cells contain recoverin while a subset of bipolar and amacrine cells contain calretinin. The calretinin expressing amacrine cells seemed to form synaptic contacts with rhodopsin containing photoreceptor cells in the OPL and contacts with melanopsin cell bodies and dendrites in the IPL. The study demonstrates the complex retinal circuitry used by the Spalax to detect light, and provides evidence for both melanopsin and non-melanopsin projecting pathways to the brain.

Life in a dark biosphere: a review of circadian physiology in "arrhythmic" environments

Most of the life with which humans interact is exposed to highly rhythmic and extremely predictable changes in illumination that occur with the daily events of sunrise and sunset. However, while the influence of the sun feels omnipotent to surface dwellers such as ourselves, life on earth is dominated, in terms of biomass, by organisms isolated from the direct effects of the sun. A limited understanding of what life is like away from the sun can be inferred from our knowledge of physiology and ecology in the light biosphere, but a full understanding can only be gained by studying animals from the dark biosphere, both in the laboratory and in their natural habitats. One of the least understood aspects of life in the dark biosphere is the rhythmicity of physiology and what it means to live in an environment of low or no rhythmicity. Here we describe methods that may be used to understand rhythmic physiology in the dark and summarise some of the studies of rhythmic physiology in "arrhythmic" environments, such as the poles, deep sea and caves. We review what can be understood about the adaptive value of rhythmic physiology on the Earth's surface from studies of animals from arrhythmic environments and what role a circadian clock may play in the dark.

Stress, adaptation, and speciation in the evolution of the blind mole rat, Spalax, in Israel

Environmental stress played a major role in the evolution of the blind mole rat superspecies Spalax ehrenbergi, affecting its adaptive evolution and ecological speciation underground. Spalax is safeguarded all of its life underground from aboveground climatic fluctuations and predators. However, it encounters multiple stresses in its underground burrows including darkness, energetics, hypoxia, hypercapnia, food scarcity, and pathogenicity. Consequently, it evolved adaptive genomic, proteomic, and phenomic complexes to cope with those stresses. Here I describe some of these adaptive complexes, and their theoretical and applied perspectives. Spalax mosaic molecular and organismal evolution involves reductions or regressions coupled with expansions or progressions caused by evolutionary tinkering and natural genetic engineering. Speciation of Spalax in Israel occurred in the Pleistocene, during the last 2.00-2.35 Mya, generating four species associated intimately with four climatic regimes with increasing aridity stress southwards and eastwards representing an ecological speciational adaptive trend: (Spalax golani, 2n=54→S. galili, 2n=52→S. carmeli, 2n=58→S. judaei, 2n=60). Darwinian ecological speciation occurred gradually with relatively little genetic change by Robertsonian chromosomal and genic mutations. Spalax genome sequencing has just been completed. It involves multiple adaptive complexes to life underground and is an evolutionary model to a few hundred underground mammals. It involves great promise in the future for medicine, space flight, and deep-sea diving.

Spectral and duration sensitivity to light-at-night in 'blind' and sighted rodent species

Light-at-night (LAN) has become a defining feature of human and animal ecosystems and may possibly compromise human and animal physiology and health. Spectral and acclimation duration (AD) sensitivity were compared between social voles (Microtus socialis) and 'blind' mole rats (Spalax ehrenbergi) in four increasing ADs (0, 1, 7 and 21 days) to LAN (1×30 min, 293 μW cm(-2)) of three different monochromatic lights [blue (479 nm), yellow (586 nm) and red (697 nm)]. Animals were sampled for urine and oxygen consumption (V(O(2))) promptly after each LAN-AD. Urine samples were analyzed for production rate, urinary 6-sulfatoxymelatonin and urinary metabolites of adrenalin and cortisol. Overall, the blue light elicited the greatest effects on the biological markers of M. socialis, whereas similar effects were detected for S. ehrenbergi in response to red light. The increasing LAN-AD resulted in a dose-dependent decrement of all markers tested, except of stress hormones, which showed a direct positive correlation with LAN-AD. Our results suggest that: (1) photoperiod is an important cue for entraining physiological functions in the 'blind' S. ehrenbergi, which is essentially characterized by red-shifted sensitivity compared with the blue-shifted sensitivity detected for the sighted counterpart species, and (2) there is a strong association between LAN of the appropriate wavelength and adrenal endocrine responses, suggesting that LAN is a potential environmental stressor.

Light, photoreceptors, and circadian clocks

Research over the past decade has focused increasingly on the photoreceptor mechanisms that regulate the circadian system in all forms of life. Some of the results to emerge are surprising. For example, the rods and cones within the mammalian eye are not required for the alignment (entrainment) of circadian rhythms to the dawn-dusk cycle. There exists a population of directly light-sensitive ganglion cells within the eye that act as brightness detectors; these regulate both circadian rhythms and melatonin synthesis. An understanding of these "circadian photoreceptor" pathways, and the features of the light environment used for entrainment, have been and will continue to be heavily dependent on the appropriate use and measurement of light stimuli. Furthermore, if results from different laboratories, or species, are to be compared in any meaningful sense, standardized methods for light measurement and manipulation need to be adopted by circadian biologists. To this end, we describe light measurement in terms of both radiometric and photometric units and consider the appropriate use of light as a stimulus in circadian experiments. In addition, the construction of action spectra has been very helpful in associating photopigments with particular responses in a broad range of photobiological systems. Because the identity of the photopigments mediating circadian responses to light are often not known, we have also taken this opportunity to provide a step-by-step approach to conducting action spectra, including the construction of irradiance response curves, the calculation of relative spectral sensitivities, photopigment template fitting, and the underlying assumptions behind this approach. The aims of this chapter are to provide an accessible introduction to photobiological methods and explain why these approaches need to be applied to the study of circadian systems.

Melanopsin: Another Way of Signaling Light

A subset of melanopsin-expressing retinal ganglion cells has been identified to be directly photosensitive (pRGCs), modulating a range of behavioral and physiological responses to light. Recent expression studies of melanopsin have provided compelling evidence that melanopsin is the photopigment of the pRGCs. However, the mechanism by which melanopsin transduces light information remains an open question. This review discusses the signaling pathways that may underlie melanopsin-dependent phototransduction in native pRGCs, as well as the many exciting challenges ahead.

Circadian genes in a blind subterranean mammal III: molecular cloning and circadian regulation of cryptochrome genes in the blind subterranean mole rat, Spalax ehrenbergi superspecies

The blind subterranean mole rat superspecies Spalax ehrenbergi is an extreme example of mammalian adaptation to life underground. Though this rodent is totally visually blind, harboring a drastically degenerated subcutaneous rudimentary eye, its daily activity rhythm is entrainable to LD cycles. This indicates that it confers light information to the clock, as has been previously shown by the authors in behavioral studies as well as by molecular analyses of its Clock/MOP3 and its three Per genes. The Cryptochrome (Cry) genes found in animals and plants act both as photoreceptors and as essential components of the negative feedback mechanism of the biological clock. To further understand the circadian system of this unique mammal, the authors cloned and characterized the open reading frame of Spalax Cry1 and Cry2. The Spalax CRY1 protein is significantly closer to the human homolog than to the mice one, in contrast to the evolutionary expectations. They have found two isoforms of Cry2 in Spalax, which differ in their 5' end of the open reading frame and defined their expression in Spalax populations. They found a large and significant excess of heterozygotes of sCry2 (sCry2L/S genotype). Both sCry1 and sCry2 mRNAs were found in the SCN, the eye, the harderian gland, as well as in a wide range of peripheral tissues. Their expression pattern under different LD conditions has also been analyzed. As was already shown for other circadian genes, despite being blind and living in darkness, the Cry genes of Spalax behave in a similar, though not identical, pattern as in sighted animals. Once again, the results indicate that the uniquely hypertrophied harderian gland of Spalax plays a key role in its circadian system.

Unusual cone and rod properties in subterranean African mole-rats (Rodentia, Bathyergidae)

We have determined the presence of spectral cone types, and the population densities of cones and rods, in subterranean mole-rats of the rodent family Bathyergidae, for which light and vision seems of little importance. Most mammals have two spectral cone types, a majority of middle- to long-wave-sensitive (L-) cones, and a minority of short-wave-sensitive (S-)cones. We were interested to see whether the subterranean bathyergids show the same pattern. In three species, Ansell's mole-rat Cryptomys anselli, the giant mole-rat Cryptomys mechowi and the naked mole-rat Heterocephalus glaber, spectral cone types and rods were assessed immunocytochemically with opsin-specific antibodies. All three species had rod-dominated retinae but possessed significant cone populations. A quantitative assessment in C. anselli and C. mechowi revealed surprisingly low photoreceptor densities of 100 000-150 000/mm(2), and high cone proportions, approximately 10% (8000-15 000/mm(2)). In all three species, the vast majority of the cones were strongly S-opsin-immunoreactive; L-opsin immunoreactivity was much fainter. In C. anselli, approximately 20% of the cones showed exclusive S-opsin label, approximately 10% exclusive L-opsin label and approximately 70% strong S-opsin and faint L-opsin double label (potential dual-pigment cones). This is the first observation in any mammal of an S-opsin dominance and low levels of L-opsin across the entire retina. It contrasts starkly with the situation in the muroid blind mole-rat Spalax ehrenbergi, which has been reported to possess L-opsin but no S-opsin. Evidently, within rodents an adaptation to subterranean life is compatible with very different spectral cone properties.

The eye of the african mole-rat Cryptomys anselli: to see or not to see?

In an attempt to clarify its possible physiological role, we studied the eye of the Zambian mole rat Cryptomys anselli by light, electron and confocal microscopy using conventional staining as well as immunolabelling with rod and cone cell markers. The small eyes of Cryptomys are located superficially and display all features typical of sighted animals: iris, pupil and well-developed lens, separating the anterior chamber and the vitreous. The retina shows a well stratified organization and the folds described in blind subterranean or nocturnal mammals were not observed. The major population of the photoreceptor cells in the Cryptomys retina consists of rod cells, again with a morphology quite similar to that found in sighted animals. The relatively short outer segments contain numerous well-stacked disks and show a strong rod-opsin as well as transducin immunoreaction. Synapses were evident in the spherules, the round basal processes of the rod cell, but they lacked the precise organization reported for sighted mammals. Cone cells were present as well, as indicated by peanut lectin staining, but no immunolabelling with polyclonal M/L-opsin antisera was detectable. The presence of cone cells was also suggested by some basal processes at the outer plexiform layer which displayed several synaptic active sites and irregular contours. While the other retinal layers also showed an organization typical of sighted mammals, there were signs of less tightly preserved morphology as well. Displaced rods and amacrine and/or ganglion cells were observed, and some sparse rod spherules penetrated into the inner nuclear layer. A major reduction was observed in the number of ganglion cells, estimated from the number of axons in the optic nerve, that was very low (approximately 1000 per retina on average) relative to sighted mammals. The data we have suggest a slow, ongoing loss of cells with ageing. Apoptotic nuclei, mainly corresponding to photoreceptor cells and ganglion cells, were detected in young individuals, and an overall reduction in the thickness of the retina was observed in older animals. The morphological data presented here allow some first speculations on the physiological role of the Cryptomys eye and will hopefully trigger detailed studies on the chronobiology and the anatomy of the retinal projections and of the visual cortex of this remarkable species.

A green cone-like pigment in the ‘blind’ mole-rat Spalax ehrenbergi: functional expression and photochemical characterization

The degenerate subcutaneous eye of the blind mole rat belonging to the Spalax ehrenbergi superspecies has been shown to contain a long wavelength sensitive (LWS) cone pigment. Baculovirus expression of this LWS pigment and subsequent IMAC purification yields a photosensitive protein, that according to absorbance maximum (530 +/- 2 nm), kinetics of late phototransitions, and transducin activation, has all characteristics of a functional green cone pigment. The absorbance spectrum of the Spalax pigment is strongly red-shifted relative to the very homologous mouse, rabbit and rat green cone pigments (508-510 nm). Also in contrast to the rodent pigments, the Spalax pigment exhibits anion-dependent spectral properties, displaying a 12 nm blue-shift upon substitution of chloride ions by nitrate ions. Finally, the slow part of the photocascade deviates in some aspects from that of sighted mammals. The possible relevance of these findings for the evolutionary adaptation of Spalax to a subterranean ecotope is discussed.

A switch from diurnal to nocturnal activity in S. ehrenbergi is accompanied by an uncoupling of light input and the circadian clock

The subterranean mole rat Spalax ehrenbergi superspecies represents an extreme example of adaptive visual and neuronal reorganization. Despite its total visual blindness, its daily activity rhythm is entrainable to light-dark cycles, indicating that it can confer light information to the clock. Although most individuals are active during the light phase under laboratory conditions (diurnal animals), some individuals switch their activity period to the night (nocturnal animals). Similar to other rodents, the Spalax circadian clock is driven by a set of clock genes, including the period (sPer) genes. In this work, we show that diurnal mole rats express the Per genes sPer1 and sPer2 with a peak during the light period. Light can synchronize sPer gene expression to an altered light-dark cycle and thereby reset the clock. In contrast, nocturnal Spalax express sPer2 in the dark period and sPer1 in a biphasic manner, with a light-dependent maximum during the day and a second light-independent maximum during the night. Although sPer1 expression remains light inducible, this is not sufficient to reset the molecular clockwork. Hence, the strict coupling of light, Per expression, and the circadian clock is lost. This indicates that Spalax can dissociate the light-driven resetting pathway from the central clock oscillator.

Non-rod, non-cone photoreception in the vertebrates

When reflected from a surface, light can provide a representation of the spatial environment, whilst gross changes in environment light can signal the time of day. The differing sensory demands of using light to detect environmental space and time appear to have provided the selection pressures for the evolution of different photoreceptor systems in the vertebrates, and probably all animals. This point has been well recognised in the non-mammals, which possess multiple opsin/vitamin A-based photoreceptor populations in a variety of sites distributed both within and outside the CNS. By contrast, eye loss in mammals abolishes all responses to light, and as a result, all photoreception was attributed to the rods and cones of the retina. However, studies over the past decade have provided overwhelming evidence that the mammalian eye contains a novel photoreceptor system that does not depend upon the input from the rods and cones. Mice with eyes but lacking rod and cone photoreceptors can still detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity, and modulate pupil size. Furthermore, action spectra for some of these responses in rodents and humans have characterised at least one novel opsin/vitamin A-based photopigment, and molecular studies have identified a number of candidate genes for this photopigment. Parallel studies in fish showing that VA opsin photopigment is expressed within sub-sets of inner retina neurones, demonstrates that mammals are not alone in having inner retinal photoreceptors. It therefore seems likely that inner retinal photoreception will be a feature of all vertebrates. Current studies are directed towards an understanding of their mechanisms, determining the extent to which they contribute to physiology and behaviour in general, and establishing how they may interact with other photoreceptors, including the rods and cones. Progress on each of these topics is moving very rapidly. As a result, we hope this review will serve as an introduction to the cascade of papers that will emerge on these topics in the next few years. We also hope to convince the more casual reader that there is much more to vertebrate photoreceptors than the study of retinal rods and cones.

Adaptive loss of ultraviolet-sensitive/violet-sensitive (UVS/VS) cone opsin in the blind mole rat (Spalax ehrenbergi)

In previous studies, fully functional rod and long-wavelength-sensitive (LWS) cone photopigments have been isolated from the eye of the subterranean blind mole rat (Spalax ehrenbergi superspecies). Spalax possesses subcutaneous atrophied eyes and lacks any ability to respond to visual images. By contrast this animal retains the ability to entrain circadian rhythms of locomotor behaviour to environmental light cues. As this is the only known function of the eye, the rod and LWS photopigments are thought to mediate this response. Most mammals are dichromats possessing, in addition to a single rod photopigment, two classes of cone photopigment, LWS and ultraviolet-sensitive/violet-sensitive (UVS/VS) with differing spectral sensitivities which mediate colour vision. In this paper we explore whether Spalax is a dichromat and has the potential to use colour discrimination for photoentrainment. Using immunocytochemistry and molecular approaches we demonstrate that Spalax is a LWS monochromat. Spalax lacks a functional UVS/VS cone photopigment due to the accumulation of several deleterious mutational changes that have rendered the gene nonfunctional. Using phylogenetic analysis we show that the loss of this class of photoreceptor is likely to have arisen from the visual ecology of this species, and is not an artefact of having an ancestor which lacked a functional UVS/VS cone photopigment. We conclude that colour discrimination is not a prerequisite for photoentrainment in this species.

Circadian genes in a blind subterranean mammal II: conservation and uniqueness of the three Period homologs in the blind subterranean mole rat, Spalax ehrenbergi superspecies

We demonstrated that a subterranean, visually blind mammal has a functional set of three Per genes that are important components of the circadian clockwork in mammals. The mole rat superspecies Spalax ehrenbergi is a blind subterranean animal that lives its entire life underground in darkness. It has degenerated eyes, but the retina and highly hypertrophic harderian gland are involved in photoperiodic perception. All three Per genes oscillate with a periodicity of 24 h in the suprachiasmatic nuclei, eye, and harderian gland and are expressed in peripheral organs. This oscillation is maintained under constant conditions. The light inducibility of sPer1 and sPer2, which are similar in structure to those of other mammals, indicates the role of these genes in clock resetting. However, sPer3 is unique in mammals and has two truncated isoforms, and its expressional analysis leaves its function unresolved. Per's expression analysis in the harderian gland suggests an important participation of this organ in the stabilization and resetting mechanism of the central pacemaker in the suprachiasmatic nuclei and in unique adaptation to life underground.

Power analysis of tests for loss of selective constraint in cave crayfish and nonphotosynthetic plant lineages

Loss of selective constraint on a gene may be expected following changes in the environment or life history that render its function unnecessary. The long-term persistence of protein-coding genes after the loss of known functional necessity can occur by chance or because of selective maintenance of an unknown gene function. The selective maintenance of an alternative gene function is not demonstrated by the failure of statistical tests to reject the hypothesis that there has been no change in the degree of constraint on the evolution of coding genes. Maintenance may be inferred, however, when power analyses of such tests demonstrate that there has been a sufficient number of nucleotide substitutions to detect the loss of selective constraint. Here, we describe a power analysis for tests of loss of constraint on protein-coding genes. The power analysis was applied to loss-of-constraint tests for opsin gene evolution in cave-dwelling crayfish and rbcL evolution in nonphotosynthetic parasitic plants. The power of previously applied tests for loss of constraint on cave crayfish opsin genes was insufficient to distinguish between chance retention and selective maintenance of opsin genes. However, the power of codon-based likelihood ratio tests for change in d(N)/d(S) (=omega) (nonsynonymous to synonymous change) did have sufficient power to detect a loss of constraint on rbcL associated with a loss of photosynthesis in most examples but failed to detect such a change in three independent lineages. We conclude that rbcL has been selectively maintained in these holoparasitic plant lineages. This conclusion suggests that either these taxa are photosynthetic for at least a part of their life or rbcL may have an unknown function in these plants unrelated to photosynthesis.

The eye lens protein alphaA-crystallin of the blind mole rat Spalax ehrenbergi: effects of altered functional constraints

The rudimentary eyes of the mole rat Spalax ehrenbergi have lost their visual function, but are still required for the control of circadian rhythms. It has previously been found that alphaA-crystallin, a major eye lens protein in other mammals, evolved much faster in the mole rat than in rodents with normal vision. Yet, although mole rat alphaA-crystallin seems superfluous as a lens protein, its rate of change is still much slower than that of pseudogenes, suggesting some remaining function. The authors therefore studied the structure and function of recombinant mole rat alphaA-crystallin. Circular dichroism (CD), tryptophan fluorescence and gel permeation analyses indicated that the overall structure and stability of mole rat alphaA-crystallin are comparable to that of rat alphaA-crystallin. However, the chaperone-like activity of mole rat alphaA-crystallin is considerably lower than that of its rat orthologue. Two-dimensional NMR spectroscopy of mole rat alphaA-crystallin suggests that this may be in part due to a diminished flexibility of the C-terminal extension, which is thought to be important for the chaperoning capacity. Overall, mole rat alphaA-crystallin appears to still be a viable protein, confirming that it has some as yet elusive role, despite the loss of its primary lens function.

The regulation of circadian clocks by light in fruitflies and mice

A circadian clock has no survival value unless biological time is adjusted (entrained) to local time and, for most organisms, the profound changes in the light environment provide the local time signal (zeitgeber). Over 24 h, the amount of light, its spectral composition and its direction change in a systematic way. In theory, all of these features could be used for entrainment, but each would be subject to considerable variation or 'noise'. Despite this high degree of environmental noise, entrained organisms show remarkable precision in their daily activities. Thus, the photosensory task of entrainment is likely to be very complex, but fundamentally similar for all organisms. To test this hypothesis we compare the photoreceptors that mediate entrainment in both flies and mice, and assess their degree of convergence. Although superficially different, both organisms use specialized (employing novel photopigments) and complex (using multiple photopigments) photoreceptor mechanisms. We conclude that this multiplicity of photic inputs, in highly divergent organisms, must relate to the complex sensory task of using light as a zeitgeber.

Biological clock in total darkness: the Clock/MOP3 circadian system of the blind subterranean mole rat

Blind subterranean mole rats retain a degenerated, subcutaneous, visually blind but functionally circadian eye involved in photoperiodic perception. Here we describe the cloning, sequence, and expression of the circadian Clock and MOP3 cDNAs of the Spalax ehrenbergi superspecies in Israel. Both genes are relatively conserved, although characterized by a significant number of amino acid substitutions. The glutamine-rich area of Clock, which is assumed to function in circadian rhythmicity, is expanded in Spalax compared with that of humans and mice, and is different in amino acid composition from that of rats. We also show that MOP3 is a bona fide partner of Spalax Clock and that the Spalax Clock/MOP3 dimer is less potent than its human counterpart in driving transcription. We suggest that this reduction in transcriptional activity may be attributed to the Spalax Clock glutamine-rich domain, which is unique in its amino acid composition compared with other studied mammalian species. Understanding Clock/MOP3 function could highlight circadian mechanisms in blind mammals and their unique pattern as a result of adapting to life underground.

Identifying the photoreceptive inputs to the mammalian circadian system using transgenic and retinally degenerate mice

The endogenous circadian clock of mammals retains synchrony with the external light:dark cycle through ocular photoreceptors. To date the identity of the photoreceptors responsible for mediating this response is unknown. This review outlines attempts using transgenic mouse models to address this deficit. Mice bearing specific inherited lesions of both rod and cone photoreceptors retain circadian photosensitivity as assessed by photoentrainment of behavioural rhythms and the light-induced suppression of pineal melatonin. These findings indicate that as yet unidentified non-rod, non-cone ocular photoreceptors are capable of contributing to circadian light responses. Nevertheless, the possibility that circadian photosensitivity is the responsibility of multiple photoreceptor classes including both rod/cone and novel photopigments remains. There is some indirect evidence in favour of this hypothesis. A definitive resolution of this issue is likely to employ comparisons of circadian action spectra in wild type and retinally degenerate mice.

The molecular genetics and evolution of red and green color vision in vertebrates

To better understand the evolution of red-green color vision in vertebrates, we inferred the amino acid sequences of the ancestral pigments of 11 selected visual pigments: the LWS pigments of cave fish (Astyanax fasciatus), frog (Xenopus laevis), chicken (Gallus gallus), chameleon (Anolis carolinensis), goat (Capra hircus), and human (Homo sapiens);and the MWS pigments of cave fish, gecko (Gekko gekko), mouse (Mus musculus), squirrel (Sciurus carolinensis), and human. We constructed these ancestral pigments by introducing the necessary mutations into contemporary pigments and evaluated their absorption spectra using an in vitro assay. The results show that the common ancestor of vertebrates and most other ancestors had LWS pigments. Multiple regression analyses of ancestral and contemporary MWS and LWS pigments show that single mutations S180A, H197Y, Y277F, T285A, A308S, and double mutations S180A/H197Y shift the lambda(max) of the pigments by -7, -28, -8, -15, -27, and 11 nm, respectively. It is most likely that this "five-sites" rule is the molecular basis of spectral tuning in the MWS and LWS pigments during vertebrate evolution.

Extraocular phototransduction and circadian timing systems in vertebrates

It is widely accepted that, for organisms with eyes, the daily regulation of circadian rhythms is made possible by light transduction through those organs. Yet, it has been demonstrated repeatedly in recent years that ocular light receptors that mediate vision, at least in mammals, are not the same photoreceptors involved in circadian regulation. Moreover, it has been recognized for many years that circadian regulation can occur in organisms without eyes. In fact, extraocular circadian phototransduction (EOCP) appears to be a phylogenetic rule for the vast majority of species. EOCP has been reported in every nonmammalian species studied to date. In mammals, however, the story is very different. This paper presents findings from studies that have examined specifically the capacity for EOCP in vertebrate species. In addition, the literature addressing noncircadian aspects of extraocular phototransduction is briefly discussed. Finally, possible mechanisms underlying EOCP are discussed, as are some of the implications of the presence, or absence, of EOCP across phylogeny.

The lens protein alpha-B-crystallin of the blind subterranean mole-rat: high homology with sighted mammals

Blind subterranean mole rats, Spalax ehrenbergi, retain a subcutaneous, degenerated eye, which is visually non-functional but which does function in circadian entrainment. Crystallins, members of the small heat shock protein family, constitute approximately 90% of the water-soluble proteins of the transparent eye lens and are crucial for its optical properties, but they are also expressed in other tissues. In our attempt to understand the role of the eye in the blind mole-rat, we now describe the cloning, sequencing, and expression of the cDNA of alpha-B-Crystallin from two species of Spalax (S. galili and S. Judaei, with diploid chromosome numbers 2n=52 and 60, respectively). Spalax alpha- B-Crystallin is highly conserved. It is expressed in many tissues of Spalax, among them Spalax eye. The sequence of the cDNA of alpha-B-Crystallin in the eye and in the heart of Spalax is identical. Further studies are essential to clarify the role of this gene in the lens of an atrophied eye of a visually blind mammal.

The mammalian photoreceptor mosaic-adaptive design

Unlike in birds and cold-blooded vertebrates' retinas, the photoreceptors of mammalian retinas were long supposed to be morphologically uniform and difficult to distinguish into subtypes. A number of new techniques have now begun to overcome the previous limitations. A hitherto unexpected variability of spectral and morphological subtypes and topographic patterns of distribution in the various retinas are being revealed. We begin to understand the design of the photoreceptor mosaics, the constraints of evolutionary history and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mammalian photoreceptor types and speculates on the likely "bottleneck-scenario" for the origin of the basic design of the mammalian retina. It then provides a brief synopsis of current data on the photoreceptors in the various mammalian orders and derives some trends for phenomena such as rod/cone dualism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (probably dichromatic) model, mammalian retinas have developed considerable radiation. Comparing the nonprimate models with the intensively studied primate model should provide us with a deeper understanding of the basic design of the mammalian retina.

Evolution of eyes

Seeing is important for most species and it has been a key selective advantage throughout evolution. Consequently, there is a remarkable diversity among types of eyes. Animals have converged on eight optical solutions for collecting and focusing light; in contrast, all eyes share the same molecular strategy for absorbing photons. Recent studies have identified similarities in the genetic information that is used in the development of eyes, leading to the hypothesis that distinctly different eye types might have had a monophyletic origin. Across many species, there is a remarkable continuity of the developmental genes that participate in the construction of similar--but not necessarily homologous--eyes.