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Heterochronic opsin expression due to early light deprivation results in drastically shifted visual sensitivity in a cichlid fish: Possible role of thyroid hormone signaling
Abstract
During early ontogeny, visual opsin gene expression in cichlids is influenced by prevailing light reg-imen. Red light, for example, leads to an early switch from the expression of short-wavelength sensitive to long-wavelength sensitive opsins. Here, we address the influence of light deprivation on opsin expression. Individuals reared in constant darkness during the first 14 days post-hatching (dph) showed a general developmental delay compared with fish reared under a 12:12 hr light-dark cycle (control group). Several characters including pigmentation patterns and eye development , appeared later in dark-reared individuals. Quantitative real-time PCR and fluorescent in situ hybridization at six time points during the 14 days period revealed that fish from the control group expressed opsin genes from 5 dph on and maintained a short-wavelength sensitive phenotype (sws1, rh2b, and rh2a). Onset of opsin expression in dark-reared Midas cichlids was delayed by 4 days and visual sensitivity rapidly progressed toward a long-wavelength sensitive phenotype (sws2b, rh2a, and lws). Shifts in visual sensitivities toward longer wavelengths are mediated by thyroid hormone (TH) in many vertebrates. Compared to control fish, dark-reared individuals showed elevated dio3 expression levels-a validated proxy for TH concentration-suggesting higher circulating TH levels. Despite decelerated overall development, ontogeny of opsin gene expression was accelerated, resulting in retinae with long-wavelength shifted predicted sensitivities compared to light-reared individuals. Indirect evidence suggests that this was due to altered TH metabolism.
... During ontogeny, many cichlid species shift opsin expression from short-to long-wavelength sensitivity [10]. Phenotypic plasticity in opsin expression seems to be rather common during this ontogenetic progression but appears to be restricted once the adult visual phenotype has been reached [5,14], but see [6,15]. Amphilophus astorquii, our study species, shows a paedomorphic visual phenotype, which might allow this species to maintain some degree of plasticity into adulthood, at least in the same direction as the ontogenetic progression (towards longer wavelengths) [4]. ...
... Expression levels of opsin genes were based on qPCR data of two technical replicates and mean threshold cycle (Ct) values were used for subsequent analysis. Before and after the 14-day experimental period, expression of cone opsins was calculated proportionally to overall cone opsin expression (incorporating qPCR efficiencies) as previously described [4,14]. For fish reared under either blue or red light for 118 and 132 days, we tested whether treatment or age has an effect on cone opsin expression by using a nonparametric equivalent to a two-way ANOVA, the Scheirer-Ray-Hare test [17]. ...
... We then calculated mean values for each of the eight sections along the dorsoventral axis. For further details on FISH methodology, see Karagic et al. [14]. Quantum catch, which represents a measure for the number of photons a visual pigment captures, was calculated for each opsin gene and light treatment based on equations from Govardovskii et al. [19]. ...
Phenotypic plasticity, particularly during development, allows organisms to rapidly adjust to different environmental conditions. Yet, it is often unclear whether the extent and direction of plastic changes are restricted by an individual's ontogeny. Many species of cichlid fishes go through ontogenetic changes in visual sensitivity, from short to long wavelengths, by switching expression of cone opsin genes crucial for colour vision. During this progression, individuals often exhibit phenotypic plasticity to the ambient light conditions. However, it is commonly assumed that once an adult visual phenotype is reached, reverting to an earlier ontogenetic state with higher sensitivity at shorter wavelengths is not common. In this study, we experimentally demonstrate that four-month-old Midas cichlid fish (Amphilophus astorquii) show plasticity in single cone opsin expression after experiencing drastic changes in light conditions. Resulting shifts of visual sensitivity occurred presumably in an adaptive direction-towards shorter or longer wavelengths when exposed to short- or long-wavelength light, respectively. Single cone opsin expression changed within only a few days and went through a transitional phase of co-expression. When the environment was experimentally enriched in long-wavelength light, the corresponding change occurred gradually along a dorsoventral gradient within the retina. This plasticity allowed individuals to revert earlier ontogenetic changes and return to a more juvenile visual phenotype demonstrating previously unrecognized insights into temporal and spatial dynamics of phenotypic plasticity of the visual system in response to ambient light.
... Thyroid hormone has also been suggested to play an important role in the adaptive divergence of visual sensitivity in Nicaraguan Midas cichlid fishes (Amphilophus citrinellus (Günther 1864)) Karagic et al. 2018). Midas cichlids are a young and small species flock comprised of at least 13 species (Torres-Dowdall and Meyer 2021). ...
... Accordingly, the spectral sensitivity of crater lake Midas cichlids has adaptively diverged from the ancestral form in the great lakes by shifting toward shorter wavelengths through changes in both components of their visual pigment, chromophore type usage and identity of opsin proteins Härer et al. 2018). There is evidence suggesting that this change might be driven by changes in the circulating TH level during development Karagic et al. 2018). ...
... Crater lake species have lower TH levels than fish from the great lakes (indirectly measured via differences in deiodinase expression) . Moreover, changes in the expression of different genes involved in TH metabolism are observed when the ontogenetic progression of spectral sensitivity is altered by manipulating light conditions (Karagic et al. 2018). Previous studies used the expression of TH-metabolizing genes as proxies for circulating TH levels, so it is unclear whether direct manipulations of TH levels can elicit a response in visual sensitivities. ...
Vision is critical for most vertebrates, including fish. One challenge that aquatic habitats pose is the high variability in spectral properties depending on depth and the inherent optical properties of the water. By altering opsin gene expression and chromophore usage, cichlid fish modulate visual sensitivities to maximize sensory input from the available light in their respective habitat. Thyroid hormone (TH) has been proposed to play a role in governing adaptive diversification in visual sensitivity in Nicaraguan Midas cichlids, which evolved in less than 4,000 generations. As suggested by indirect measurements of TH levels (i.e., expression of deiodinases), populations adapted to short wavelength light in clear lakes have lower TH levels than ones inhabiting turbid lakes enriched in long‐wavelength light. We experimentally manipulated TH levels by exposing two‐week‐old Midas cichlids to exogenous TH or a TH‐inhibitor and measured opsin gene expression and chromophore usage (via cyp27c1 expression). Whereas exogenous TH induces long‐wavelength sensitivity by changing opsin gene expression and chromophore usage in a concerted manner, TH‐inhibited fish exhibit a visual phenotype with sensitivities shifted to shorter‐wavelengths. Tinkering with TH levels in eyes results in concerted phenotypic changes that can provide a rapid mechanism of adaptation to novel light environments. This article is protected by copyright. All rights reserved
... Laboratory-reared Midas cichlids originated from a mixed-sex stock tank comprising wild-caught individuals collected in Lake Nicaragua in 2010 and 2013. For the experiment, conducted in 2017, embryos from an A. citrinellus brood were divided into 12 groups 2 days post-fertilization (n ¼ 10 each); six were placed in a 12 L : 12 D cycle and six in constant darkness (for details, see [13]). Two weeks after hatching, the percentages of surviving fish and of those with spinal deformities were quantified. ...
... Melatonin is involved in the regulation of growth and development, and minimal light thresholds are needed for proper larval development in most fishes [18]. Indeed, dark-raised A. citrinellus have a disrupted thyroid metabolism [13], a downstream target of melatonin [18]. Not surprisingly, surgical removal of the pineal gland has been reported to cause spinal deformities in vertebrates [23]. ...
Only few fish species have successfully colonized subterranean habitats, but the underlying biological constraints associated with this are still poorly understood. Here, we investigated the influence of permanent darkness on spinal-column development in one species (Midas cichlid, Amphilophus citrinellus) with no known cave form, and one (Atlantic molly, Poecilia mexicana) with two phylogenetically young cave forms. Specifically, fish were reared under a normal light : dark cycle or in permanent darkness (both species). We also surveyed wild-caught cave and surface ecotypes of P. mexicana. In both species, permanent darkness was associated with significantly higher rates of spinal deformities (especially in A. citrinellus). This suggests strong developmental (intrinsic) constraints on the successful colonization of subterranean environments in teleost fishes and might help explain the relative paucity of cave-adapted lineages. Our results add depth to our understanding of the aspects of selection driving trait divergence and maintaining reproductive isolation in cave faunas.
... Moreover, this effect was validated by rearing Midas cichlids in water supplemented with TH, finding that the progression in opsin gene expression got accelerated , as it occurs in other vertebrates (Roberts et al. 2006;Cheng et al. 2009). Disruption of the TH metabolism, by rearing Midas cichlids in complete darkness, also resulted in elevated levels of TH and acceleration of developmental progression from short-to long-wavelength shifted sensitivities (Karagic et al. 2018). Ongoing experiments are aiming to determine if the retina regulates TH metabolism independently of the rest of the body, which will decouple the effect of TH on eye development from overall development. ...
The Nicaraguan Midas cichlid species complex is a natural experiment where fish from a large source population from turbid and shallow great lakes very recently (<20,000 years ago) colonized eight small crater lakes. The colonizers experienced completely novel environments in the clear and deep calderas. So far, 13 Midas cichlid species have been described, but more genetic clusters were identified. Although some of these species arose in allopatry, many more evolved in the absence of barriers to gene flow within two crater lakes. They contain small radiations of four and six endemics, respectively. These radiations constitute one of the few generally accepted empirical examples for sympatric speciation making them an ideal system for studying repeated evolution of adaptations and speciation at different levels of biological organization, including the genome level. Diversification occurred repeatedly in parallel including body morphology, coloration, color perception, and trophic structures such as pharyngeal jaws and hypertrophied lips. Additionally, parallel speciation happened in the two small crater lake radiations, where ecomorphologically similar species evolved repeatedly. Genomic differentiation associated with oligogenic traits (e.g., hypertrophic lips and coloration) is shallow, remaining polymorphisms, but much higher for polygenic traits (e.g., body shape and pharyngeal jaw morphology) that distinguish new species.
... TH regulatory pathways act as mechanistic links between environmental change and phenotypic development (Lema 2020). For example, changes in feeding conditions, temperature, and light intensity cause alterations in the TH levels in fish (Eales et al. 1982;Karagic et al. 2018;Leiner & Mackenzie 2003), which can affect phenotypic expression (Lema 2020). It has been shown that plasticity in endocrine signaling may play an important role in evolutionary changes in different taxa of teleost fish (Kitano et al. 2010;Lema 2020;Prazdnikov and Shkil 2019a;Shkil and Smirnov 2015). ...
Male guppies display an outstanding diversity of color patterns which is formed as a result of a complex interplay between sexual selection, predation, and other environmental factors. The heterogeneity of the environment affects the variability of ornamental traits in male guppies through genotype–environment interaction. Thyroid hormones (THs) are important regulators of the ontogeny of fish and serve as a link between environmental changes and phenotypic development. However, the role of THs in the formation of a variety of color patterns in male guppies remained poorly understood. In this work, an experimental assessment of the effect of THs on the variability of ornamental traits in Poecilia wingei males was carried out. The fish were reared from birth to the initial stages of the formation of melanistic elements in males and until the final formation of the color pattern; they were subjected to different hormonal regimes: euthyroidism (natural TH status), hyperthyroidism (high TH level, at a triiodo-L-thyronine concentration of 0.15 μg/mL), and hypothyroidism (TH-deficiency, at a thiourea concentration of 0.025%). Alterations in the TH status caused changes in the timing and rate of the development of coloration and affected the transformation of various elements of the color pattern in males. These changes led to an increase in phenotypic variability and the appearance of ornamental traits in the male color patterns that were characteristic of closely related species of Poecilia. Thus, the data obtained indicate a potentially important role of thyroid hormones in the diversification of guppy color patterns and open up new prospects for studying the role of endocrine regulatory mechanisms in the adaptive evolution of poeciliid fish.
... Expression levels of the four candidate genes (unk, odam, rpfA, scppS) as weil as two housekeeping genes (actinR, twinfilin; Gunter et al. 2013) were analyzed by qPCR with the GoTaq qPCR System (Promega, Madison, WI) as described in Karagic et al. (2018). Significant differences in each genes' expression values were compared for each species pair using individuals as replicates with a t-test (a = 0.05). ...
The two toothed jaws of cichlid fishes provide textbook examples of convergent evolution. Tooth phenotypes such as enlarged molar-like teeth used to process hard-shelled molluscs have evolved numerous times independently during cichlid diversification. While the ecological benefit of molar-like teeth to crush prey is known, it is unclear whether the same molecular mechanisms underlie these convergent traits. To identify genes involved in the evolution and development of enlarged cichlid teeth, we performed RNA-seq on the serially homologous toothed oral and pharyngeal jaws as well as the fourth toothless gill arch of Astatoreochromis alluaudi. We identified 27 genes that are highly upregulated on both tooth-bearing jaws compared to the toothless gill arch. Most of these genes have never been reported to play a role in tooth formation. Two of these genes (unk, rpfA) are not found in other vertebrate genomes but are present in all cichlid genomes. They also cluster genomically with two other highly expressed tooth genes (odam, scpp5) that exhibit conserved expression during vertebrate odontogenesis. Unk and rpfA were confirmed via in situ hybridization to be expressed in developing teeth of Astatotilapia burtoni. We then examined expression of the cluster's four genes in six evolutionarily independent and phylogenetically disparate cichlid species pairs each with a large- and a small-toothed species. Odam and unk commonly and scpp5 and rpfA always showed higher expression in larger-toothed cichlid jaws. Convergent trophic adaptations across cichlid diversity are associated with the repeated developmental deployment of this genomic cluster containing conserved and novel cichlid-specific genes.
... In addition to shifts that completely replace one cone opsin with another, partial expression shifts can occur, resulting in coexpression of two cone opsins in a single photoreceptor. [24,[77][78][79]. This coexpression has been found in both African and New World cichlids and produces cone sensitivities that are intermediate between the coexpressed pair [78]. ...
The family Cichlidae contains approximately 2000 species that live in diverse freshwater habitats including murky lakes, turbid rivers, and clear lakes from both the Old and New Worlds. Their visual systems are similarly diverse and have evolved specific sensitivities that differ along several axes of variation. Variation in cornea and lens transmission affect which wavelengths reach the retina. Variation in photoreceptor number and distribution affect brightness sensitivity, spectral sensitivity and resolution. Probably their most dynamic characteristic is the variation in visual pigment peak sensitivities. Visual pigments can be altered through changes in chromophore, opsin sequence and opsin expression. Opsin expression varies by altering which of the seven available cone opsins in their genomes are turned on. These opsins can even be coexpressed to produce seemingly infinitely tunable cone sensitivities. Both chromophore and opsin expression can vary on either rapid (hours or days), slower (seasonal or ontogenetic) or evolutionary timescales. Such visual system shifts have enabled cichlids to adapt to different habitats and foraging styles. Through both short term plasticity and longer evolutionary adaptations, cichlids have proven to be ecologically successful and an excellent model for studying organismal adaptation.
... Escobar-Camacho et al. (2016) found that the same three opsins were expressed in all of three different studied Amazonian cichlids, suggesting that Amazonian cichlids generally have a limited opsin repertoire. Although sampled representatives of Central American lineages expressed the same three cone opsins in our study, they appear to be generally less constrained in their visual repertoire as recent studies found more visual diversity among them, with at least some of them also expressing sws1, sws2b and rh2b in certain habitat types or developmental stages Karagic et al., 2018). ...
Cichlid fishes’ famous diversity in body coloration is accompanied by a highly diverse and complex visual system. Although cichlids possess an unusually high number of seven cone opsin genes, they express only a subset of these during their ontogeny, accounting for their astonishing interspecific variation in visual sensitivities. Much of this diversity is thought to have been shaped by natural selection as cichlids inhabit a variety of habitats with distinct light environments. Also, sexual selection might have contributed to the observed visual diversity, and sexual dimorphism in coloration potentially co‐evolved with sexual dimorphism in opsin expression. We investigated sex‐specific opsin expression of several cichlids from Africa and the Neotropics and collected and integrated datasets on sex‐specific body coloration, species‐specific visual sensitivities, lens transmission and habitat light properties for some of them. We comparatively analyzed this wide range of molecular and ecological data, illustrating how integrative approaches can address specific questions on the factors and mechanisms driving diversification, and the evolution of cichlid vision in particular. We found that both sexes expressed opsins at the same levels ‐ even in sexually dimorphic cichlid species – which argues against coevolution of sexual dichromatism and differences in sex‐specific visual sensitivity. Rather, a combination of environmental light properties and body coloration shaped the diversity in spectral sensitivities among cichlids. We conclude that although cichlids are particularly colorful and diverse and often sexually dimorphic, it would appear that natural rather than sexual selection is a more powerful force driving visual diversity in this hyper‐diverse lineage.
Cichlid fishes show remarkable variation in visual sensitivities through differential expression of seven cone opsin genes. Many species undergo spectral sensitivity shifts from shorter to longer wavelengths as they develop from larvae to adults. However, while some species retain larval-like short wavelength sensitivities, others show adult-like longer wavelength sensitivities throughout life. The riverine cichlid, Astatotilapia burtoni, shows a single cone progression from ultraviolet to violet to blue sensitivity, while their long wavelength double cones maintain green and red sensitivities throughout life. To identify mechanisms that regulate these sensitivities, we asked whether thyroid hormone (TH) or light environment can drive shifts. We find that developmental treatment with TH can speed shifts to longer wavelength sensitivity, but only in single cones. TH inhibition can short wavelength shift adult opsin expression. Exposure to light regimes containing UV wavelengths induce short wavelength shifts in single cones early in development. None of the treatments produces double cone shifts or significant expression of the shortest wavelength double cone opsin, rh2b, although we detect no cis-regulatory variation. This suggests that while single cones show both TH and light plasticity, A. burtoni double cones have lost this plasticity, perhaps through changes in trans-acting opsin regulation.
Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970–80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.
Highlights
• Heterochrony research focuses on comparative morphology but lags on mechanistic understanding.
• We propose integrating the different levels of study into a single analysis flow: from morphology to cells to molecules and genes, using modern techniques.
Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, Maurolicus spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides’ habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.
Circadian clocks regulate various aspects of photoreceptor physiology, but their contribution to photoreceptor development and function is unclear. Cone photoreceptors are critical for color vision. Here, we define the molecular function of circadian activity within cone photoreceptors and reveal a role for the clock genes Bmal1 and Per2 in regulating cone spectral identity. ChIP analysis revealed that BMAL1 binds to the promoter region of the thyroid hormone (TH)-activating enzyme type 2 iodothyronine deiodinase (Dio2) and thus regulates the expression of Dio2. TH treatment resulted in a partial rescue of the phenotype caused by the loss of Bmal1, thus revealing a functional relationship between Bmal1 and Dio2 in establishing cone photoreceptor identity. Furthermore, Bmal1 and Dio2 are required to maintain cone photoreceptor functional integrity. Overall, our results suggest a mechanism by which circadian proteins can locally regulate the availability of TH and influence tissue development and function.
Midas cichlid fish are a Central American species flock containing 13 described species that has been dated to only few thousand years old, a historical timescale infrequently associated with speciation. Their radiation involved the colonization of several clear water crater lakes from two turbid great lakes. Therefore, Midas cichlids have been subjected to widely varying photic conditions during their radiation. Being a primary signal relay for information from the environment to the organism, the visual system is under continuing selective pressure and a prime organ system for accumulating adaptive changes during speciation, particularly in the case of dramatic shifts in photic conditions. Here, we characterize the full visual system of Midas cichlids at organismal and genetic levels, to determine what types of adaptive changes evolved within the short time span of their radiation. We show that Midas cichlids have a diverse visual system with unexpectedly high intra-and interspecific variation in color vision sensitivity and lens transmittance. Midas cichlid populations in the clear crater lakes have convergently evolved visual sensitivities shifted towards shorter wavelengths compared to the ancestral populations from the turbid great lakes. This divergence in sensitivity is driven by changes in chromophore usage, differential opsin expression, opsin coexpression, and to a lesser degree by opsin coding sequence variation. The visual system of Midas cichlids has the evolutionary capacity to rapidly integrate multiple adaptations to changing light environments. Our data may indicate that, in early stages of divergence, changes in opsin regulation could precede changes in opsin coding sequence evolution.
Subterranean biodiversity in Europe is spectacularly rich, with the Western Balkans being home to about 400 cave species, representing the highest number of species per area worldwide [1]. Nonetheless, cave fishes, which are the most commonly found vertebrates in underground habitats [2], have not been described from Europe so far [3]. Here, we report the first European record of a cave fish population, a loach of the genus Barbatula (Figure 1), found in the Danube–Aach system, an underground karst water system in Southern Germany [4]. The fish exhibit traits typically observed in organisms adapted to subterranean life including reduced eyes and pale body coloration [5]. The newly discovered population also represents globally the northernmost cave fish found so far. The geological history of the region implies that the Danube–Aach system was colonized post-glacially. A recent origin of the cave fish is supported by genetic analyses, because the subterranean population shares COI gene haplotypes with adjacent surface stone loach (Barbatula barbatula) populations (Figure 1D). Nonetheless, population genetic analyses based on microsatellites indicated that cave fish are genetically isolated from populations in surface habitats (Figure 1E) and exhibit reduced genetic variability. Hence, the newly discovered European cave loaches do not represent individuals displaced from surface populations, but they follow a unique evolutionary trajectory towards cave life.
Flatfish have the most extreme asymmetric body morphology of vertebrates. During metamorphosis, one eye migrates to the contralateral side of the skull, and this migration is accompanied by extensive craniofacial transformations and simultaneous development of lopsided body pigmentation. The evolution of this developmental and physiological innovation remains enigmatic. Comparative genomics of two flatfish and transcriptomic analyses during metamorphosis point to a role for thyroid hormone and retinoic acid signaling, as well as phototransduction pathways. We demonstrate that retinoic acid is critical in establishing asymmetric pigmentation and, via cross-talk with thyroid hormones, in modulating eye migration. The unexpected expression of the visual opsins from the phototransduction pathway in the skin translates illumination differences and generates retinoic acid gradients that underlie the generation of asymmetry. Identifying the genetic underpinning of this unique developmental process answers long-standing questions about the evolutionary origin of asymmetry, but it also provides insight into the mechanisms that control body shape in vertebrates.
The formation of species in the absence of geographic barriers (i.e. sympatric speciation) remains one of the most controversial topics in evolutionary biology. While theoretical models have shown that this most extreme case of primary divergence-with-gene-flow is possible, only a handful of accepted empirical examples exist. And even for the most convincing examples uncertainties remain; complex histories of isolation and secondary contact can make species falsely appear to have originated by sympatric speciation. This alternative scenario is notoriously difficult to rule out. Midas cichlids inhabiting small and remote crater lakes in Nicaragua are traditionally considered to be one of the best examples of sympatric speciation and lend themselves to test the different evolutionary scenarios that could lead to apparent sympatric speciation since the system is relatively small and the source populations known. Here we reconstruct the evolutionary history of two small-scale radiations of Midas cichlids inhabiting crater lakes Apoyo and Xiloá through a comprehensive genomic data set. We find no signs of differential admixture of any of the sympatric species in the respective radiations. Together with coalescent simulations of different demographic models our results support a scenario of speciation that was initiated in sympatry and does not result from secondary contact of already partly diverged populations. Furthermore, several species seem to have diverged simultaneously, making Midas cichlids an empirical example of multispecies outcomes of sympatric speciation. Importantly, however, the demographic models strongly support an admixture event from the source population into both crater lakes shortly before the onset of the radiations within the lakes. This opens the possibility that the formation of reproductive barriers involved in sympatric speciation was facilitated by genetic variants that evolved in a period of isolation between the initial founding population and the secondary migrants that came from the same source population. Thus, the exact mechanisms by which these species arose might be different from what had been thought before.
Background
The visual system is important for animals for mate choice, food acquisition, and predator avoidance. Animals possessing a visual system can sense particular wavelengths of light emanating from objects and their surroundings and perceive their environments by processing information contained in these visual perceptions of light. Visual perception in individuals varies with the absorption spectra of visual pigments and the expression levels of opsin genes, which may be altered according to the light environments. However, which light environments and the mechanism by which they change opsin expression profiles and whether these changes in opsin gene expression can affect light sensitivities are largely unknown. This study determined whether the light environment during growth induced plastic changes in opsin gene expression and behavioral sensitivity to particular wavelengths of light in guppies (Poecilia reticulata). ResultsIndividuals grown under orange light exhibited a higher expression of long wavelength-sensitive (LWS) opsin genes and a higher sensitivity to 600-nm light than those grown under green light. In addition, we confirmed that variations in the expression levels of LWS opsin genes were related to the behavioral sensitivities to long wavelengths of light. Conclusions
The light environment during the growth stage alters the expression levels of LWS opsin genes and behavioral sensitivities to long wavelengths of light in guppies. The plastically enhanced sensitivity to background light due to changes in opsin gene expression can enhance the detection and visibility of predators and foods, thereby affecting survival. Moreover, changes in sensitivities to orange light may lead to changes in the discrimination of orange/red colors of male guppies and might alter female preferences for male color patterns.
Light affects animal physiology and behavior more than simply through classical visual, image-forming pathways. Nonvisual photoreception regulates numerous biological systems, including circadian entrainment, DNA repair, metabolism , and behavior. However, for the majority of these processes, the photoreceptive molecules involved are unknown. Given the diversity of photophysiological responses, the question arises whether a single photopigment or a greater diversity of proteins within the opsin superfamily detect photic stimuli. Here, a functional genomics approach identified the full complement of photopigments in a highly light-sensitive model vertebrate, the zebrafish (Danio rerio), and characterized their tissue distribution, expression levels, and biochemical properties. The results presented here reveal the presence of 42 distinct genes encoding 10 classical visual photopigments and 32 nonvisual opsins, including 10 novel opsin genes comprising four new pigment classes. Consistent with the presence of light-entrainable circadian oscillators in zebrafish, all adult tissues examined expressed two or more opsins, including several novel opsins. Spectral and electrophysiological analyses of the new opsins demonstrate that they form functional photopigments, each with unique chromophore-binding and wavelength specificities. This study has revealed a remarkable number and diversity of photopigments in zebrafish, the largest number so far discovered for any vertebrate. Found in amphibians, reptiles, birds, and all three mammalian clades, most of these genes are not restricted to teleosts. Therefore, nonvisual light detection is far more complex than initially appreciated, which has significant biological implications in understanding photoreception in vertebrates.
Background
Central American crater lake cichlid fish of the Midas species complex (Amphilophus spp.) are a model system for sympatric speciation and fast ecological diversification and specialization. Midas cichlids have been intensively analyzed from an ecological and morphological perspective. Genomic resources such as transcriptomic and genomic data sets, and a high-quality draft genome are available now. Many ecologically relevant species- specific traits and differences such as pigmentation and cranial morphology arise during development. Detailed descriptions of the early development of the Midas cichlid in particular, will help to investigate the ontogeny of species differences and adaptations.
Results
We describe the embryonic and larval development of the crater lake cichlid, Amphilophus xiloaensis, until seven days after fertilization. Similar to previous studies on teleost development, we describe six periods of embryogenesis - the zygote, cleavage, blastula, gastrula, segmentation, and post-hatching period. Furthermore, we define homologous stages to well-described teleost models such as medaka and zebrafish, as well as other cichlid species such as the Nile tilapia and the South American cichlid Cichlasoma dimerus. Key

morphological differences between the embryos of Midas cichlids and other teleosts are highlighted and discussed, including the presence of adhesive glands and different early melanophore patterns, as well as variation in developmental timing.
Conclusions
The developmental staging of the Midas cichlid will aid researchers in the comparative investigation of teleost ontogenies. It will facilitate comparative developmental biological studies of Neotropical and African cichlid fish in particular. In the past, the species flocks of the African Great Lakes have received the most attention from researchers, but some lineages of the 300–400 species of Central American lakes are fascinating model systems for adaptive radiation and rapid phenotypic evolution. The availability of genetic resources, their status as a model system for evolutionary research, and the possibility to perform functional experiments including transgenesis makes the Midas cichlid complex a very attractive model for evolutionary-developmental research.
Vision frequently mediates critical behaviours, and photoreceptors must respond to the light available to accomplish these tasks. Most photoreceptors are thought to contain a single visual pigment, an opsin protein bound to a chromophore, which together determine spectral sensitivity. Mechanisms of spectral tuning include altering the opsin, changing the chromophore and incorporating pre-receptor filtering. A few exceptions to the use of a single visual pigment have been documented in which a single mature photoreceptor coexpresses opsins that form spectrally distinct visual pigments, and in these exceptions the functional significance of coexpression is unclear. Here we document for the first time photoreceptors coexpressing spectrally distinct opsin genes in a manner that tunes sensitivity to the light environment. Photoreceptors of the cichlid fish, Metriaclima zebra, mix different pairs of opsins in retinal regions that view distinct backgrounds. The mixing of visual pigments increases absorbance of the corresponding background, potentially aiding the detection of dark objects. Thus, opsin coexpression may be a novel mechanism of spectral tuning that could be useful for detecting prey, predators and mates. However, our calculations show that coexpression of some opsins can hinder colour discrimination, creating a trade-off between visual functions.
The mesopelagic zone is a visual scene continuum in which organisms have developed various strategies to optimize photon capture. Here, we used light microscopy, stereology-assisted retinal topographic mapping, spectrophotometry and microspectrophotometry to investigate the visual ecology of deep-sea bioluminescent sharks [four etmopterid species (Etmopterus lucifer, E. splendidus, E. spinax and Trigonognathus kabeyai) and one dalatiid species (Squaliolus aliae)]. We highlighted a novel structure, a translucent area present in the upper eye orbit of Etmopteridae, which might be part of a reference system for counterillumination adjustment or acts as a spectral filter for camouflage breaking, as well as several ocular specialisations such as aphakic gaps and semicircular tapeta previously unknown in elasmobranchs. All species showed pure rod hexagonal mosaics with a high topographic diversity. Retinal specialisations, formed by shallow cell density gradients, may aid in prey detection and reflect lifestyle differences; pelagic species display areae centrales while benthopelagic and benthic species display wide and narrow horizontal streaks, respectively. One species (E. lucifer) displays two areae within its horizontal streak that likely allows detection of conspecifics' elongated bioluminescent flank markings. Ganglion cell topography reveals less variation with all species showing a temporal area for acute frontal binocular vision. This area is dorsally extended in T. kabeyai, allowing this species to adjust the strike of its peculiar jaws in the ventro-frontal visual field. Etmopterus lucifer showed an additional nasal area matching a high rod density area. Peak spectral sensitivities of the rod visual pigments (λmax) fall within the range 484-491 nm, allowing these sharks to detect a high proportion of photons present in their habitat. Comparisons with previously published data reveal ocular differences between bioluminescent and non-bioluminescent deep-sea sharks. In particular, bioluminescent sharks possess higher rod densities, which might provide them with improved temporal resolution particularly useful for bioluminescent communication during social interactions.
Exposure to appropriate levels of thyroid hormones (THs) at the right time is of key importance for normal development in all vertebrates. Type 3 iodothyronine deiodinase (D3) is the prime TH-inactivating enzyme and its expression is highest in the early stages of vertebrate development, implying it may be necessary to shield developing tissues from overexposure to THs. We used antisense morpholino-knockdown to examine the role of D3 during early development in zebrafish. Zebrafish possess two D3 genes, dio3a and dio3b. Here we show that both genes are expressed during development and both contribute to in vivo D3 activity. However, dio3b mRNA levels in embryos are higher and the effects of dio3b knockdown on D3 activity and on the resulting phenotype are more severe. D3 knockdown induced an overall delay in development, as determined by measurements of otic vesicle length, eye and ear size, and body length. The time of hatching was also severely delayed in D3-knockdown embryos. Importantly, we also observed a severe disturbance of several aspects of development. Swim bladder development and inflation was aberrant as was the development of liver and intestine. Furthermore, D3-knockdown larvae spent significantly less time moving, and both embryos and larvae exhibited perturbed escape responses, suggesting that D3 knockdown affects muscle development and/or functioning. These data indicate that D3 is essential for normal zebrafish embryonic and early larval development and show the value of morpholino knockdown in this model to further elucidate the specific role of D3 in some aspects of vertebrate development.
Light is a key environmental factor that synchronizes all life-stages of fish, from embryo development to sexual maturation. The underwater photo-environment is complex since light characteristics (i.e. intensity, photoperiod and spectrum) depend on the absorbance properties of the water column. The aim of this paper is to review the effects of artificial lighting conditions on the performance, development and welfare of some fish larvae of commercial interest. Reviewed results show that larvae were significantly affected by light characteristics. For example, European sea bass and sole larvae achieved the best performance, and showed fastest development and lowest degree of deformity under a light/dark cycle using blue light (half-peak bandwidth = 435–500 nm), conditions which were the closest to their natural aquatic environment. However, constant light (LL) or constant darkness (DD) was shown to negatively affect normal larval development and resulted in increased malformations and poor survival in most of the studied species. Similar results have been observed in other fish larvae such as Atlantic cod, which performed better under short wavelengths (blue and green). These findings highlight the role of lighting conditions during the early development of fish larvae and should be taken into account for the optimization of rearing protocols in fish hatcheries as juvenile supply is one of the main production bottlenecks.
The genetic mechanisms underlying regressive evolution-the degeneration or loss of a derived trait-are largely unknown, particularly for complex structures such as eyes in cave organisms. In several eyeless animals, the visual photoreceptor rhodopsin appears to have retained functional amino acid sequences. Hypotheses to explain apparent maintenance of function include weak selection for retention of light-sensing abilities and its pleiotropic roles in circadian rhythms and thermotaxis. In contrast, we show that there has been repeated loss of functional constraint of rhodopsin in amblyopsid cavefishes, as at least three cave lineages have independently accumulated unique loss-of-function mutations over the last 10.3 Mya. Although several cave lineages still possess functional rhodopsin, they exhibit increased rates of nonsynonymous mutations that have greater effect on the structure and function of rhodopsin compared to those in surface lineages. These results indicate that functionality of rhodopsin has been repeatedly lost in amblyopsid cavefishes. The presence of a functional copy of rhodopsin in some cave lineages is likely explained by stochastic accumulation of mutations following recent subterranean colonization.
Background
Phenotypic evolution may occur through mutations that affect either the structure or expression of protein-coding genes. Although the evolution of color vision has historically been attributed to structural mutations within the opsin genes, recent research has shown that opsin regulatory mutations can also tune photoreceptor sensitivity and color vision. Visual sensitivity in African cichlid fishes varies as a result of the differential expression of seven opsin genes. We crossed cichlid species that express different opsin gene sets and scanned their genome for expression Quantitative Trait Loci (eQTL) responsible for these differences. Our results shed light on the role that different structural, cis-, and trans-regulatory mutations play in the evolution of color vision.
Results
We identified 11 eQTL that contribute to the divergent expression of five opsin genes. On three linkage groups, several eQTL formed regulatory “hotspots” associated with the expression of multiple opsins. Importantly, however, the majority of the eQTL we identified (8/11 or 73%) occur on linkage groups located trans to the opsin genes, suggesting that cichlid color vision has evolved primarily via trans-regulatory divergence. By modeling the impact of just two of these trans-regulatory eQTL, we show that opsin regulatory mutations can alter cichlid photoreceptor sensitivity and color vision at least as much as opsin structural mutations can.
Conclusions
Combined with previous work, we demonstrate that the evolution of cichlid color vision results from the interplay of structural, cis-, and especially trans-regulatory loci. Although there are numerous examples of structural and cis-regulatory mutations that contribute to phenotypic evolution, our results suggest that trans-regulatory mutations could contribute to phenotypic divergence more commonly than previously expected, especially in systems like color vision, where compensatory changes in the expression of multiple genes are required in order to produce functional phenotypes.
The West Indian Ophidiiform fish are characterized by the occurrence of several troglobitic species. They are Ogilbia (=Typhliasina pearsei (Yucatan), Lucifuga subterruneus, L. dentatus and L. simile (Cuba), and L. spelaeotes (Babama Islands). The surface relatives, e.g. Ogilbia spp., prefer lightpoor zones. This is shown by a large lens and a retina almost exclusively consistin of rods. L. dentatus, L. subterraneus, and O. pearsei possess extremely reduced eyes. The eye of L. selaeotes has morphologically an intermediate position. In correlation to varying eye size a different number of rods may be developed. Furthermore in some specimens the lens starts degenerating. Contrar to the extremely reduced cave forms all specimens of L. spelaeotes can still perceive light with tleir eyes. The different degrees of reduction in the West Indian Ophidiiform cave fish probably reflect varying phylogenetic ages. A possible date of origin of L. spelaeotes could be at the beginning or the end of the last glaciation.
The eyes of neonate individuals of the live-bearing L. dentatus and O. pearsei possess a lens and a retina rudiment. No fundamental structural and histological differences between both species were found at this stage.
The Trinidadian pike cichlid (Crenicichla frenata) is a major predator of the guppy (Poecilia reticulata), a model system for visual ecology research, and visual predation by the pike cichlid is known to select for male guppies with reduced short-wavelength reflectance. However, an early study of the pike cichlid's visual system suggested a lack of short-wavelength-sensitive cone photoreceptors, a surprising finding as many African cichlids have highly developed short-wavelength vision. In this study, we found evidence for only four expressed cone opsins (LWS, RH2a, SWS2a, and SWS2b), plus one pseudogene (RH2b). Taken together with our microspectrophotometry data, which revealed the presence of three types of cone photoreceptor, including one sensitive to short-wavelength light, this would indicate a broader spectral capacity than previously believed from earlier visual studies of this fish. Relative to the highly diverse African cichlids, however, this Neotropical cichlid appears to have a greatly reduced opsin complement, reflecting both gene loss along the Neotropical lineage (lacking functional RH2b and, possibly, SWS1 opsins) and gene duplication within the African clade (which possesses paralogous RH2aα and RH2aβ opsins). Molecular evolutionary analyses show that positive selection has shaped the SWS2b and RH1 opsins along the Neotropical lineage, which may be indicative of adaptive evolution to alter nonspectral aspects of opsin biology. These results represent the first molecular evolutionary study of visual pigments in a Neotropical cichlid and thus provide a foundation for further study of a morphologically and ecologically diverse clade that has been understudied with respect to the link between visual ecology and diversification.
The early morphogenesis of the degenerate eyes of the Mexican cave fish Astyanax fasciatus and of its conspecific epigean ancestor has been studied comparatively using light- and electron-microscopical techniques; the transcription of the opsin gene has been analysed during early ontogeny in both populations by in situ hybridization. The opsin protein is an integral component of the disk membranes of the photoreceptor cells. In epigean specimens, its expression is correlated with the beginning of outer segment formation on the third day of development. Morphogenesis of the cave fish eye is similar to that of the epigean eye until end of the third day. However, eye growth and morphogenesis of the retina are delayed and sporadic cell death occurs in all retinal layers at the beginning of the second day. Retinal cytodifferentiation breaks off at the point of outer segment formation. Cave specimens are not able to develop regular outer segments at any stage, but the opsin gene is nevertheless expressed in the outer nuclear layer of the developing retina for a limited period of time. On the basis of the comparative morphology and transcriptional studies of epigean and cave specimens, it is suggested that the eye regression of cave fish is primarily the result of mutations of developmental control genes and not of structural genes.
A series of trials was conducted to determine the effect of extended periods of light on the growth, survival, feeding pattern and daily feed consumption of barramundi, Lates calcarifer, larvae and juveniles. Larvae 2–10 days old grew progressively faster under conditions of 8, 16 and 24 h light per day; survival rates did not differ between the treatments. Larvae 8–20 days old grew significantly slower in 8 h light than in 16 and 24 h light; survival rates did not differ between the treatments. For juveniles initially 11–12 mm total length, there was no significant difference in growth or survival rates in either 12, 18, or 24 h light. Juvenile barramundi exposed to a 12L/12D photoperiod fed continuously during daylight, and ceased feeding in darkness. Under continuous daylight, juvenile barramundi fed throughout the normal daytime period, but ceased feeding at a time corresponding to the onset of darkness; feeding started again near midnight. Daily food consumption for 34-mm fish was approximately 40% more in continuous light than in 12L/12D photoperiod. The results indicate that extended light increases growth rates of barramundi during their first 8–10 days, but thereafter it becomes less important and confers no advantage after metamorphosis.
1. The role of phenotypic plasticity in evolution has historically been a contentious issue because of debate over whether plasticity shields genotypes from selection or generates novel opportunities for selection to act. Because plasticity encompasses diverse adaptive and non-adaptive responses to environmental variation, no single conceptual framework adequately predicts the diverse roles of plasticity in evolutionary change. 2. Different types of phenotypic plasticity can uniquely contribute to adaptive evolution when populations are faced with new or altered environments. Adaptive plasticity should promote establishment and persistence in a new environment, but depending on how close the plastic response is to the new favoured phenotypic optimum dictates whether directional selection will cause adaptive divergence between populations. Further, non-adaptive plasticity in response to stressful environments can result in a mean phenotypic response being further away from the favoured optimum or alternatively increase the variance around the mean due to the expression of cryptic genetic variation. The expression of cryptic genetic variation can facilitate adaptive evolution if by chance it results in a fitter phenotype. 3. We conclude that adaptive plasticity that places populations close enough to a new phenotypic optimum for directional selection to act is the only plasticity that predictably enhances fitness and is most likely to facilitate adaptive evolution on ecological time-scales in new environments. However, this type of plasticity is likely to be the product of past selection on variation that may have been initially non-adaptive. 4. We end with suggestions on how future empirical studies can be designed to better test the importance of different kinds of plasticity to adaptive evolution.
Analysis with DNA-microrrays and real time PCR show that several genes involved in the thyroid hormone cascade, such as deiodinase 2 and 3 (Dio2 and Dio3) are differentially regulated by the circadian clock and by changes of the ambient light. The expression level of Dio2 in adult rats (2-3 months of age) kept continuously in darkness is modulated by the circadian clock and is up-regulated by 2 fold at midday. When the diurnal ambient light was on, the expression level of Dio2 increased by 4-8 fold and a consequent increase of the related protein was detected around the nuclei of retinal photoreceptors and of neurons in inner and outer nuclear layers. The expression level of Dio3 had a different temporal pattern and was down-regulated by diurnal light. Our results suggest that DIO2 and DIO3 have a role not only in the developing retina but also in the adult retina and are powerfully regulated by light. As the thyroid hormone is a ligand-inducible transcription factor controlling the expression of several target genes, the transcriptional activation of Dio2 could be a novel genomic component of light adaptation.
Visual ecology is the study of how animals use visual systems to meet their ecological needs, how these systems have evolved, and how they are specialized for particular visual tasks. This book provides the first up-to-date synthesis of the field to appear in more than three decades. Featuring some 225 illustrations, including more than 140 in color, spread throughout the text, the book begins by discussing the basic properties of light and the optical environment. It then looks at how photoreceptors intercept light and convert it to usable biological signals, how the pigments and cells of vision vary among animals, and how the properties of these components affect a given receptor's sensitivity to light. The book goes on to examine how eyes and photoreceptors become specialized for an array of visual tasks, such as navigation, evading prey, mate choice, and communication. A timely and much-needed resource for students and researchers alike, the book also includes a glossary and a wealth of examples drawn from the full diversity of visual systems.
Only few fish species have successfully colonized subterranean habitats, but the underlying biological constraints associated with this are still poorly understood. Here, we investigated the influence of permanent darkness on spinal-column development in one species (Midas cichlid, Amphilophus citrinellus) with no known cave form, and one (Atlantic molly, Poecilia mexicana) with two phylogenetically young cave forms. Specifically, fish were reared under a normal light : dark cycle or in permanent darkness (both species). We also surveyed wild-caught cave and surface ecotypes of P. mexicana. In both species, permanent darkness was associated with significantly higher rates of spinal deformities (especially in A. citrinellus). This suggests strong developmental (intrinsic) constraints on the successful colonization of subterranean environments in teleost fishes and might help explain the relative paucity of cave-adapted lineages. Our results add depth to our understanding of the aspects of selection driving trait divergence and maintaining reproductive isolation in cave faunas.
Colonization of novel habitats is typically challenging to organisms. In the initial stage after colonization, approximation to fitness optima in the new environment can occur by selection acting on standing genetic variation, modification of developmental patterns or phenotypic plasticity. Midas cichlids have recently colonized crater Lake Apoyo from great Lake Nicaragua. The photic environment of crater Lake Apoyo is shifted towards shorter wavelengths compared to great Lake Nicaragua and Midas cichlids from both lakes differ in visual sensitivity. We investigated the contribution of ontogeny and phenotypic plasticity in shaping the visual system of Midas cichlids after colonizing this novel photic environment. To this end, we measured cone opsin expression both during development and after experimental exposure to different light treatments. Midas cichlids from both lakes undergo ontogenetic changes in cone opsin expression but visual sensitivity is consistently shifted towards shorter wavelengths in crater lake fish, which leads to a paedomorphic retention of their visual phenotype. This shift might be mediated by lower levels of thyroid hormone in crater lake Midas cichlids (measured indirectly as dio2 and dio3 gene expression). Exposing fish to different light treatments revealed that cone opsin expression is phenotypically plastic in both species during early development, with short and long wavelength light slowing or accelerating ontogenetic changes, respectively. Notably, this plastic response was maintained into adulthood only in the derived crater lake Midas cichlids. We conclude that the rapid evolution of Midas cichlids’ visual system after colonizing crater Lake Apoyo was mediated by a shift in visual sensitivity during ontogeny and was further aided by phenotypic plasticity during development. This article is protected by copyright. All rights reserved.
African cichlids are an exemplary system to study organismal diversity and rapid speciation. Species differ in external morphology including jaw shape and body coloration, but also differ in sensory systems including vision. All cichlids have seven cone opsin genes with species differing broadly in which opsins are expressed. The differential opsin expression results in closely related species with substantial differences in spectral sensitivity of their photoreceptors. In this work, we take a first step in determining the genetic basis of opsin expression in cichlids. Using a second generation cross between two species with different opsin expression patterns, we make a conservative estimate that short wavelength opsin expression is regulated by a few loci. Genetic mapping in 96 F2 hybrids provides clear evidence of a cis-regulatory region for SWS1 opsin that explains 34% of the variation in expression between the two species. Additionally, in-situ hybridization has shown that SWS1 and SWS2B opsins are coexpressed in individual single cones in the retinas of F2 progeny. Results from this work will contribute to a better understanding of the genetic architecture underlying opsin expression. This knowledge will help answer long-standing questions about the evolutionary processes fundamental to opsin expression variation and how this contributes to adaptive cichlid divergence.
Ecological and behavioural constraints play a major role in shaping the visual system of different organisms. In the mesopelagic zone of the deep- sea, between 200 and 1000 m, very low intensities of downwelling light remain, creating one of the dimmest habitats in the world. This ambient light is, however, enhanced by a multitude of bioluminescent signals emitted by its inhabitants, but these are generally dim and intermittent. As a result, the visual system of mesopelagic organisms has been pushed to its sensitivity limits in order to function in this extreme environment. This review covers the current body of knowledge on the visual system of one of the most abundant and intensely studied groups of mesopelagic fishes: the lanternfish (Myctophidae). We discuss how the plasticity, performance and novelty of its visual adaptations, compared with other deep-sea fishes, might have contributed to the diversity and abundance of this family.
This article is part of the themed issue ‘Vision in dim light’.
Vision is a critical sense for organismal survival with visual sensitivities strongly shaped by the environment. Some freshwater fishes with a Gondwanan origin are distributed in both South American rivers including the Amazon, as well as African rivers and lakes. These different habitats likely required adaptations to murky and clear environments. In this study, we compare the molecular basis of Amazonian and African cichlid fishes' visual systems. We used next generation sequencing of genomes and retinal transcriptomes to examine three Amazonian cichlid species. Genome assemblies revealed six cone opsin classes (SWS1, SWS2B, SWS2A, RH2B, RH2A, LWS) and rod opsin (RH1). However, the functionality of these genes varies across species with different pseudogenes found in different species. Our results support evidence of an RH2A gene duplication event that is shared across both cichlid groups, but which was probably followed by gene conversion. Transcriptome analyses show that Amazonian species mainly express three opsin classes (SWS2A, RH2A and LWS) which likely are a good match to the long wavelength oriented light environment of the Amazon basin. Furthermore, analysis of amino acid sequences suggest that the short wavelength sensitive genes (SWS2B, SWS2A) may be under selective pressures in order to shift their spectral properties to a longer wavelength visual palette. Our results agree with the 'sensitivity hypothesis' where the light environment causes visual adaptation. Amazonian cichlid visual systems are likely adapting through gene expression, gene loss, and possibly spectral tuning of opsin sequences. Such mechanisms may be shared across the Amazonian fish fauna. This article is protected by copyright. All rights reserved.
Postnatal development of rat visual cortical functions was studied by recording extracellularly from the primary visual cortex of 22 animals ranging in age from postnatal day 17 (P17) to P45. We found that in the youngest animals (P17-P19) all visual cortical functions tested were immature. Selectivity for orientation and movement direction of visual stimuli was almost absent, most cells received binocular input and their mean receptive field size was 5-6 times the adult size. Visual acuity was half its adult value. These functional properties developed gradually during the following weeks and by P45 they were all adult-like. This functional development is affected by manipulations of the visual input such as dark rearing (DR) and monocular deprivation (MD). DR prevented the normal postnatal maturation of visual cortical functions: in P60 rats, dark reared from birth, their visual cortical functions resembled those of P19-P21 rats. MD from P15 to P45 resulted in a dramatic shift of the ocular dominance distribution (ODD) in favour of the open eye and in a loss of visual acuity for the deprived eye. To determine the sensitive period of rat visual cortex to MD (critical period) we evaluated the shift in ODD of visual cortical neurones in rats that were subjected to the progressive delay of the onset of fixed MD period (10 days). Our results show that the critical period begins around the end of the third postnatal week, peaks between the fourth and fifth week and starts to decline from the end of the fifth week
1. Kittens were visually deprived by suturing the lids of the right eye for various periods of time at different ages. Recordings were subsequently made from the striate cortex, and responses from the two eyes compared. As previously reported, monocular eye closure during the first few months of life causes a sharp decline in the number of cells that can be influenced by the previously closed eye.2. Susceptibility to the effects of eye closure begins suddenly near the start of the fourth week, remains high until some time between the sixth and eighth weeks, and then declines, disappearing finally around the end of the third month. Monocular closure for over a year in an adult cat produces no detectable effects.3. During the period of high susceptibility in the fourth and fifth weeks eye closure for as little as 3-4 days leads to a sharp decline in the number of cells that can be driven from both eyes, as well as an over-all decline in the relative influence of the previously closed eye. A 6-day closure is enough to give a reduction in the number of cells that can be driven by the closed eye to a fraction of the normal. The physiological picture is similar to that following a 3-month monocular deprivation from birth, in which the proportion of cells the eye can influence drops from 85 to about 7%.4. Cells of the lateral geniculate receiving input from a deprived eye are noticeably smaller and paler to Nissl stain following 3 or 6 days' deprivation during the fourth week.5. Following 3 months of monocular deprivation, opening the eye for up to 5 yr produces only a very limited recovery in the cortical physiology, and no obvious recovery of the geniculate atrophy, even though behaviourally there is some return of vision in the deprived eye. Closing the normal eye, though necessary for behavioural recovery, has no detectable effect on the cortical physiology. The amount of possible recovery in the striate cortex is probably no greater if the period of eye closure is limited to weeks, but after a 5-week closure there is a definite enhancement of the recovery, even though it is far from complete
Animals vary in their sensitivities to different wavelengths of light. Sensitivity differences can have fitness implications in terms of animals' ability to forage, find mates and avoid predators. As a result, visual systems are likely selected to operate in particular lighting environments and for specific visual tasks. This review focuses on cichlid vision, as cichlids have diverse visual sensitivities, and considerable progress has been made in determining the genetic basis for this variation. We describe both the proximate and ultimate mechanisms shaping cichlid visual diversity using the structure of Tinbergen's four questions. We describe 1) the molecular mechanisms that tune visual sensitivities including changes in opsin sequence and expression; 2) the evolutionary history of visual sensitivity across the African cichlid flocks; 3) the ontological changes in visual sensitivity and how modifying this developmental program alters sensitivities among species; and 4) the fitness benefits of spectral tuning mechanisms with respect to survival and mating success. We further discuss progress to unravel the gene regulatory networks controlling opsin expression and suggest that a simple genetic architecture contributes to the lability of opsin gene expression. Finally, we identify unanswered questions including whether visual sensitivities are experiencing selection, and whether similar spectral tuning mechanisms shape visual sensitivities of other fishes. This article is protected by copyright. All rights reserved.
The present study has been designed to examine the influence of pinealectomy (PX) and melatonin (Mel) treatment employed in PX-zed rats, on the proliferation of thyroid follicular cells (TFC). The rats of Group I were submitted to sham-PX, the animals of Group II were subjected to PX, as were the rats of Group III, which, additionally, were given Mel treatment (25 μg, once daily, in late afternoon subcutaneous injections - between 16:00 and 18:00 for two months, beginning at the 9th week after PX. In order to evaluate the mitotic activity, the metaphase-arrest technique was used. Pinealectomy brought about a statistically significant increase of the mean mitotic activity rate (MMAR) (p < 0.001), being assessed four months after the surgery. Melatonin treatment partially reversed the effect of PX (p < 0.05).
Critical behaviors such as predation and mate choice often depend on vision. Visual systems are sensitive to the spectrum of light in their environment, which can vary extensively both within and among habitats. Evolutionary changes in spectral sensitivity contribute to divergence and speciation. Spectral sensitivity of the retina is primarily determined by visual pigments, which are opsin proteins bound to a chromophore. We recently discovered that photoreceptors in different regions of the retina, which view objects against distinct environmental backgrounds, coexpress different pairs of opsins in an African cichlid fish, Metriaclima zebra. This coexpression tunes the sensitivity of the retinal regions to the corresponding backgrounds and may aid detection of dark objects, such as predators. Although intraretinal regionalization of spectral sensitivity in many animals correlates with their light environments, it is unknown whether variation in the light environment induces developmentally plastic alterations of intraretinal sensitivity regions. Here, we demonstrate with fluorescent in situ hybridization and qPCR that the spectrum and angle of environmental light both influence the development of spectral sensitivity regions by altering the distribution and level of opsins across the retina. Normally M. zebra coexpresses LWS opsin with RH2Aα opsin in double cones of the ventral but not the dorsal retina. However, when illuminated from below throughout development, adult M. zebra coexpressed LWS and RH2Aα in double cones both dorsally and ventrally. Thus, environmental background spectra alter the spectral sensitivity pattern that develops across the retina, potentially influencing behaviors and related evolutionary processes such as courtship and speciation. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
This paper presents a method for the analysis of ranked data arising from completely randomized factorial designs. The procedure, which is an extension of the Kruskal-Wallis ranks test, allows for the calculation of interaction effects and linear contrasts. A Monte Carlo study of the convergence of the test and a worked example are presented.
The mechanisms underlying natural phenotypic diversity are key to understanding evolution and speciation. Cichlid fishes are
among the most speciose vertebrates and an ideal model for identifying genes controlling species differences. Cichlids have
diverse visual sensitivities that result from species expressing subsets of seven cichlid cone opsin genes. We previously
identified a quantitative trait locus (QTL) that tunes visual sensitivity by varying SWS2A (short wavelength sensitive 2A) opsin expression in a genetic cross between two Lake Malawi cichlid species. Here, we identify
Rx1 (retinal and anterior neural fold homeobox) as the causative gene for the QTL using fine mapping and RNAseq in retinal transcriptomes.
Rx1 is differentially expressed between the parental species and correlated with SWS2A expression in the F2 progeny. Expression of Rx1 and SWS2A is also correlated in a panel of 16 Lake Malawi cichlid species. Association mapping in this panel identified a 413-bp deletion
located 2.5-kb upstream of the Rx1 translation start site that is correlated with decreased Rx1 expression. This deletion explains 62% of the variance in SWS2A expression across 53 cichlid species in 29 genera. The deletion occurs in both the sand and rock-dwelling cichlid clades,
suggesting that it is an ancestral polymorphism. Our finding supports the hypothesis that mixing and matching of ancestral
polymorphisms can explain the diversity of present day cichlid phenotypes.
Lanternfishes are one of the most abundant groups of mesopelagic fishes in the world's oceans and play a critical role in biomass vertical turnover. Despite their importance, very little is known about their physiology and how they use their sensory systems to survive in the extreme conditions of the deep-sea. In this study, we provide a comprehensive description of the general morphology of the myctophid eye, based on the analysis of 53 different species in order to better understand their visual capabilities. Results confirm that myctophids possess several visual adaptations for dim-light conditions, including enlarged eyes, an aphakic gap, a tapetum lucidum, and a pure rod retina with high densities of long photoreceptors. Two novel retinal specialisations were also discovered. The first specialisation is a fundal pigmentation in adult eyes, found within an isolated retinal region (typically central retina) composed of modified pigment epithelial cells, which we hypothesize to be the remnant of a more pronounced visual specialisation important in larval stages. The second specialisation is an aggregation of extracellular microtubular-like structures found within the sclerad region of the inner nuclear layer of the retina. We hypothesize that the marked interspecific differences in the hypertrophy of these microtubular-like structures may be related to inherent, differences in visual function. A general interspecific variability in other parts of the eye is also revealed and examined in this study. The contribution of both ecology and phylogeny to the evolution of ocular specialisations and vision in dim light are discussed. J. Comp. Neurol., 2013. © 2013 Wiley Periodicals, Inc.
Thyroid hormone profiles and 5′-monodeiodinase activity were determined in tilapia at different stages of early development. The results showed that both T4 and T3 were present in significant amounts in fertilized eggs. There was a steady decrease in both T4 and T3 levels during embryonic development. The levels continued to decline after hatching until around 7 days later when most of the yolk had been absorbed. The T4 level started to rise then, suggesting that the larval thyroid had begun to produce T4 at this time, which coincided with the period of faster growth of the larvae. The T3 level remained fairly constant until around 20 days after which it rose significantly.
In vitro determination of 5′-monodeiodinase activity (5′-D activity) in the whole-body homogenates of larvae showed that the enzymatic conversion of T4 to T3 was not detectable in eggs and 3-day-old larvae but detected in 5-day-old and older larvae. There was a gradual increase in the Vmax as development proceeded indicating increasing 5′-D activity during larval development. The Km values did not differ significantly in the different stages of development. These results are discussed in relation to the growth and development of the larvae.
Thyroid hormone (TH) and retinoic acid (RA) are powerful modulators of photoreceptor differentiation during vertebrate retinal development. In the embryos and young juveniles of salmonid fishes and rodents, TH induces switches in opsin expression within individual cones, a phenomenon that also occurs in adult rodents following prolonged (12 week) hypothyroidism. Whether changes in TH levels also modulate opsin expression in the differentiated retina of fish is unknown. Like TH, RA is essential for retinal development, but its role in inducing opsin switches, if any, has not been studied. Here, we investigate the action of TH and RA on single cone opsin expression in juvenile rainbow trout, zebrafish and killifish, and on the absorbance of visual pigments in rainbow trout and zebrafish. Prolonged TH exposure increased the wavelength of maximum absorbance (λmax ) of the rod, and the medium (M, green) and long (L, red) wavelength visual pigments in all fish species examined. However, unlike the opsin switch that occurred following TH exposure in the single cones of small juvenile rainbow trout (alevin), opsin expression in large juvenile rainbow trout (smolt), zebrafish or killifish remained unchanged. RA did not induce any opsin switches nor change the visual pigment absorbance of photoreceptors. Neither ligand altered cone photoreceptor densities. We conclude that RA has no effect on opsin expression or visual pigment properties in the differentiated retina of these fishes. In contrast, TH affected both single cone opsin expression and visual pigment absorbance in the rainbow trout alevin but only visual pigment absorbance in the smolt, and in zebrafish. The latter results could be explained by a combination of opsin switches and chromophore shifts from vitamin A1 to A2. J. Comp. Neurol., 2013. © 2013 Wiley Periodicals, Inc.
A class of univariate rank tests based on multiresponse permutation procedures is introduced. Asymptotic equivalence is established
between one member of this class and the Kruskal-Wallis test. Simulated power comparisons for location shift detection indicate
that another member of the class provides superior detection efficiency for location shifts of bimodal distributions and of
heavy tailed unimodal distributions. Furthermore, both tests possess substantial computational advantages over other members
of the class.
Although several studies have shown that ultraviolet (UV) wavelengths are important in naturally occurring, visually guided behaviours of vertebrates, the function of the UV cone in such behaviours is unknown. Here, I used thyroid hormone to transform the UV cones of young rainbow trout into blue cones, a phenomenon that occurs naturally as the animal grows, to test whether the resulting loss of UV sensitivity affected the animal's foraging performance on Daphnia magna, a prey zooplankton. The distances and angles at which prey were located (variables that are known indicators of foraging performance) were significantly reduced for UV knock-out fish compared with controls. Optical measurements and photon-catch calculations revealed that the contrast of Daphnia was greater when perceived by the visual system of control versus that of thyroid-hormone-treated fish, demonstrating that the UV cone enhanced the foraging performance of young rainbow trout. Because most juvenile fishes have UV cones and feed on zooplankton, this finding has wide implications for understanding the visual ecology of fishes. The enhanced target contrast provided by UV cones could be used by other vertebrates in various behaviours, including foraging, mate selection and communication.
Objective:
Troglomorphic fishes provide excellent comparative models for studying eye evolution. We describe the gross and microscopic anatomy of ocular structures of the depigmented, blind cichlid, Lamprologus lethops, and its putative sister species, Lamprologus tigripictilis collected from the lower Congo River in the Democratic Republic of Congo.
Procedures:
Both species were fixed, paraffin-sectioned and stained. Immunohistochemical staining for rhodopsin markers was also performed.
Results:
The globe in L. lethops is smaller than its sighted congener and recessed beneath bone and skin. The scleral profile maintains a wrinkled spherical shape, and the choroid is occupied by adipose tissue containing no rete mirabilis. The globe in L. lethops is foreshortened in the anterior-posterior dimension and deviated dorsally toward the midline with no extraocular muscles. At the posterior pole of the globe, there is an open periocular space containing no cell bodies. In L. tigripictilis, no choroidal adipose tissue is seen and a rete mirabilis is present. The retina of L. lethops is thinner compared with L. tigripictilis. Both species have scleral cartilage and fully developed lenses. Rhodopsin is present in the inner and outer segments of both species.
Conclusions:
Ocular adaptations evolve over time as a response to a life in darkness. Combining ocular anatomy, developmental data, and genetics will lead to insights about evolution in these fishes and contribute to understanding how ocular evolution works in other vertebrates.
Eggs of Pacific halibut were incubated under various environmental conditions. Optimum hatching occurred over a temperature range from 6 C to 8 C, whereas temperatures of 3, 10, and 11 C were lethal. Development time from fertilization to 50% hatching varied from 250 h (9 C) to 320 h (6.5 C). Salinity effects on hatching were not as critical as temperature, as long as eggs were floating during the incubation period. Light intensity between 5 and 15 lux did not affect hatching success, but high light intensity (15 lux) and red and blue light (5 lux) produced high levels of larval abnormality. Simulated transport of unfertilized eggs indicated that the eggs can be safely moved and that fertilization rate is acceptable during the first 12 h after collection.
The early development of the zebra fish embryo, Brachydanio rerio (Hamilton) was studied under thermostatically controlled temperatures ranging from 13° to 35°C. Regular and successive cleavages and a distinct continuity of morphological features proceed regularly between 23° and 34°C. Embryos kept at temperatures above or below these limits did not complete embryogenesis. Embryogenesis up to late high blastula proceeded at the same rate of development over the temperature range of 24°C to 32°C.Time-lapse cinephotomicrography was utilized in a study of development from the one-celled blastodisc to the closure of the blastopore, incubated in a modified Rose culture chamber. The number of abnormal embryos was constant over the range of temperatures from 23° to 34°C.
1. Marine organisms reared in tanks, especially larval fish, are almost certainly deprived of sufficient sensory input for the proper development of the sense organs and their associated areas in the central nervous system. Input is also inadequate for the development of learned behavioural and physiological responses.
2. Input of a deleterious kind, as a result of social stress in crowded conditions, may also be present, leading to abnormal behaviour and undesirable morphological consequences. Specific inhibitors may also retard the growth of smaller individuals.
The production of phenotype is regulated by differential gene expression. However, the regulators of gene expression need not all reside within the embryo. Environmental factors, such as temperature, photoperiod, diet, population density, or the presence of predators, can produce specific phenotypes, presumably by altering gene-expression patterns. The field of ecological developmental biology seeks to look at development in the real world of predators, competitors, and changing seasons. Ecological concerns had played a major role in the formation of experimental embryology, and they are returning as the need for knowledge about the effects of environmental change on embryos and larvae becomes crucial. This essay reviews some of the areas of ecological developmental biology, concentrating on new studies of Amphibia and Homo.
The influence of temperature and light intensity on the development of Atlantic halibut larvae (Hippoglossus hippoglossus) was studied during the resorption of the yolk sac. Holding at three different temperatures (2°C, 6°C and 10°C) was investigated together with four light intensities (0, 3, 30, 300 lux) at 6°C. The most common abnormality observed was a mouth deformity. The percent of normal larvae was significantly lower at 10°C than at 6°C and 2°C. Total darkness gave a significantly higher percent of normal larvae than any of the other three light regimes. There was no significant effect of light on survival during the resorption of the yolk sac.
This study investigates how the characteristics (spectrum and photoperiod) of artificial light affect European sea bass eggs and larvae from − 1 to 40 days post-hatching. Fertilised eggs and larvae were reared under five different light treatments: 12L:12D red light (LDR; half-peak bandwidth = 641–718 nm), 12L:12D blue light (LDB; half-peak bandwidth = 435–500 nm), 12L:12D broad-spectrum white light (LDW; 367 < λ < 1057 nm), 24L:0D broad-spectrum white light (LL) and 0L:24D (DD). The results showed that total length at day post-hatching 40 was significantly larger in larvae reared under LDB (15.4 ± 0.6 mm) and LL (15.2 ± 0.6 mm) than in larvae reared under LDR (11.7 ± 0.7 mm). Overall wet weight was highest under LDB (21.6 ± 2.02 mgr) and lowest in LDR larvae (13.6 ± 1.48 mgr). Yolk sac and oil globule absorption occurred more slowly in LDR and DD larvae, while LDB larvae developed their fin, teeth and swim bladder significantly earlier than the rest of the groups. DD larvae were unable to capture food and mortality was 100% by day post-hatching 18, while LDR larvae did not feed on rotifers, but fed on Artemia from day post-hatching 16 onwards. The best survival was obtained with the LL treatment, although significantly more problems with swim bladder development and lower jaw malformations were also identified in this group. In summary, these results highlight the key role of the light spectrum and photoperiod for European sea bass larvae, the best performance being achieved under the light conditions that best approached those of their natural aquatic environment (LDB). These findings should be considered when designing rearing protocols for larvae in aquaculture.
Transient physostomes often fail to complete initial swim bladder inflation in culture and display reduced survival and growth. Three experiments were conducted in replicate 200-l tanks to determine the effects of photoperiod and light intensity on initial swim bladder inflation, post-inflation viability (surviving larvae with inflated swim bladders) and growth in striped trumpeter (Latris lineata) larvae. Both photoperiod and light intensity were found to affect initial swim bladder inflation, growth, survival and post-inflation viability of striped trumpeter larvae. Higher initial swim bladder inflation was promoted by providing a dark-phase before and during initial swim bladder inflation. Swim-up behaviour where larvae gulped air at the water surface to fill their swim bladder was predominantly observed during the dark-phase. In Experiment 1, a 12L:12D photoperiod was inferior to either a 18L:6D or 24L:0D photoperiod for larval growth. In Experiment 2, initial swim bladder inflation was higher in larvae reared under a light intensity of 4 μmol s−1 m−2 compared to 40 μmol s−1 m−2. From Experiment 3, a 18L:6D photoperiod provided higher post-inflation viability than either 24L:0D, or a photoperiod combination of 24L:0D from stocking, changing to 21L:3D at the onset of initial swim bladder inflation. However, different optimal photoperiods for initial swim bladder inflation (18L:6D) and survival (24L:0D) lowered post-inflation viability in striped trumpeter. The results suggest a photoperiod of 18L:6D with a light intensity of 4 μmol s−1 m−2 is the best combination for post-inflation viability.
Opsin gene sequences were first reported in the 1980s. The goal of that research was to test the hypothesis that human opsins were members of a single gene family and that variation in human color vision was mediated by mutations in these genes. While the new data supported both hypotheses, the greatest contribution of this work was, arguably, that it provided the data necessary for PCR-based surveys in a diversity of other species. Such studies, and recent whole genome sequencing projects, have uncovered exceptionally large opsin gene repertoires in ray-finned fishes (taxon, Actinopterygii). Guppies and zebrafish, for example, have 10 visual opsin genes each. Here we review the duplication and divergence events that have generated these gene collections. Phylogenetic analyses revealed that large opsin gene repertories in fish have been generated by gene duplication and divergence events that span the age of the ray-finned fishes. Data from whole genome sequencing projects and from large-insert clones show that tandem duplication is the primary mode of opsin gene family expansion in fishes. In some instances gene conversion between tandem duplicates has obscured evolutionary relationships among genes and generated unique key-site haplotypes. We mapped amino acid substitutions at so-called key-sites onto phylogenies and this exposed many examples of convergence. We found that dN/dS values were higher on the branches of our trees that followed gene duplication than on branches that followed speciation events, suggesting that duplication relaxes constraints on opsin sequence evolution. Though the focus of the review is opsin sequence evolution, we also note that there are few clear connections between opsin gene repertoires and variation in spectral environment, morphological traits, or life history traits.
While the reproductive and thyroidal systems are extensively studied in fish, they are largely studied in isolation from one another, but there is evidence supporting cross-regulation between these two systems. To better understand hormone action and the potential cross-regulation between estrogen and thyroid hormones, we examined gene expression changes in estrogen receptor (ER) and thyroid receptor (TR) subtypes and key enzymes responsible for the local synthesis and availability of estrogen and thyroid hormones (aromatase B and deiodinase, respectively) in sexually regressed, adult, male goldfish in response to 3 days waterborne exposures to 17β-estradiol (E2; 1 nM), triiodothyronine (T3; 20 and 100 nM), and co-treatments thereof. Treatments with E2 alone did not effect ER subtype transcripts in the liver, telencephalon, or testis; however, in the testis, 1 nM T3 decreased ERα and ERβ1 and co-treatments of T3 and E2 decreased ERβ1 levels. TRα-1 and TRβ transcripts were not auto-regulated by T3 or cross-regulated by E2. Although deiodinase type I levels were also unaffected, deiodinase type II decreased in response to T3 treatments. Liver deiodinase type III transcripts increased in response to T3 treatments, while E2 exhibited antagonistic effects on this T3-mediated induction. These results provide novel evidence of cross-talk between the reproductive and thyroid endocrine axes in a model teleost.