A New Blind Cave Fish Population of Genus Astyanax: Geography, Morphology and Behavior

A new population of blind, cave dwelling tetra fish of the genus Astyanax was discovered in Granadas Cave, in the Balsas drainage, southern México. All blind Mexican tetras previously described are from Tampico and San Luis Potosí, northern México. The discovery of a new blind morph thus represents an independent colonization and convergent adaptation to the cave environment by this fish. Individuals of this population display variability of their troglomorphic features. Some individuals presented asymmetrical degeneration of the eyes, where one was normal, but the other somewhat reduced in size and complexity. Loss of pigmentation and eye reduction, although sometimes correlated, were not always linked; reduced eyes were found on pigmented fish and unpigmented fish often possessed normal eyes. Some individuals had reduced lens size or an absence of lens altogether. Retina is highly modified with photoreceptors sometimes absent. Eye reduction was correlated with a diminished size of the optic lobes and an increase of the prosencephalon. Modifications of the skull involve closing in of the circumorbital series of bones. Certain aspects of behavior are also modified.     

Early and late changes in Pax6 expression accompany eye degeneration during cavefish development

We have compared Pax6 expression during embryonic development in the eyed surface form (surface fish) and several different eyeless cave forms (cavefish) of the teleost Astyanax mexicanus. Despite lacking functional eyes as adults, cavefish embryos form small optic primordia, which later arrest in development and show various degrees of eye degeneration. The pattern of Pax6 mRNA expression was modified early and late during cavefish development. In early surface fish embryos, two bilateral Pax6 expression domains are present in the anterior neural plate, which extend across the midline and fuse to form the forebrain and optic primordia. In cavefish embryos, these Pax6 domains are diminished in size and remain separated, resulting in an anterior gap in Pax6 expression and presumably the formation of smaller optic primordia. The anterior gap in Pax6 expression was confirmed by double staining for Pax6 and distalless-3 mRNA, which marks the anterior margin of the neural plate and is unaltered in cavefish. Similar anterior gaps in Pax6 expression occurred in independently derived cavefish populations, suggesting that they are important in eye degeneration. Later during surface fish development, Pax6 protein is expressed in the cornea, lens, and ganglion and amacrine cells of the neural retina. Pax6 expression was gradually reduced during cavefish lens development, concomitant with lens arrest and degeneration, and was absent in the corneal epithelium, which does not differentiate in cavefish. In contrast, Pax6 expression in the retinal ganglion and amarcine cells is unmodified in cavefish, despite retarded retinal development. The results suggest that changes in Pax6 expression are involved in the evolution of cavefish eye degeneration.     

Evidence for multiple genetic forms with similar eyeless phenotypes in the blind cavefish, Astyanax mexicanus

A diverse group of animals has adapted to caves and lost their eyes and pigmentation, but little is known about how these animals and their striking phenotypes have evolved. The teleost Astyanax mexicanus consists of an eyed epigean form (surface fish) and at least 29 different populations of eyeless hypogean forms (cavefish). Current alternative hypotheses suggest that adaptation to cave environments may have occurred either once or multiple times during the evolutionary history of this species. If the latter is true, the unique phenotypes of different cave-dwelling populations may result from convergence of form, and different genetic changes and developmental processes may have similar morphological consequences. Here we report an analysis of variation in the mitochondrial NADH dehydrogenase 2 (ND2) gene among different surface fish and cavefish populations. The results identify a minimum of two genetically distinctive cavefish lineages with similar eyeless phenotypes. The distinction between these divergent forms is supported by differences in the number of rib-bearing thoracic vertebrae in their axial skeletons. The geographic distribution of ND2 haplotypes is consistent with roles for multiple founder events and introgressive hybridization in the evolution of cave-related phenotypes. The existence of multiple genetic lineages makes A. mexicanus an excellent model to study convergence and the genes and developmental pathways involved in the evolution of the eye and pigment degeneration.     

Eyeless Mexican Cavefish Save Energy by Eliminating the Circadian Rhythm in Metabolism

The eyed surface form and eyeless cave form of the Mexican tetra Astyanax mexicanus experience stark differences in the daily periodicities of light, food and predation, factors which are likely to have a profound influence on metabolism. We measured the metabolic rate of Pachón cave and surface fish at a fixed swimming speed under light/dark and constant dark photoperiods. In constant darkness surface forms exhibited a circadian rhythm in metabolism with an increase in oxygen demand during the subjective daytime, whereas cave forms did not. The lack of circadian rhythm in metabolism leads to a 27% energy savings for Pachón cave fish compared to surface fish when comparing both forms in their natural photoperiods. When surface forms were tested under constant dark conditions they expended 38% more energy than cave forms under equivalent conditions. Elimination of the circadian rhythm in metabolism may be a general feature of animals that live in perpetually dark food-limited environments such as caves or the deep sea.     

Developmental constraints in regressive evolution: studies of the expression of the γs-crystallin gene in the developing lens of cave-dwelling Astyanax fasciatus (Cuvier, 1819) (Teleostei, Characidae) by in situ hybridization

The early morphogenesis of the lens and the expression of the γs-crystallin gene was studied in epigean Astyanax fasciatus and its cave-dwelling derivative. At early stages, the lens of the cave fish develops in a way that is similar to the epigean form. Later, the developmental timing is delayed and growth ceases in the cave-fish lens. With the beginning of cytodifferentiation, the development of the lens breaks down. Crystallin lens fibres are not produced at any time and the γs-crystallin gene, which is transcribed during a limited period in the lens of epigean fishes, is not active in cave specimens. This study confirms earlier immunofluorescence observations that demonstrated the lack of crystallin proteins in the cave-fish lens, but is in contrast to results on the blind mole rat, which showed a persistence of functioning crystallins in the degenerated lens of this species. The significance of developmental constraints in regressive evolution is discussed.     

Cavefish as a model system in evolutionary developmental biology

The Mexican tetra Astyanax mexicanus has many of the favorable attributes that have made the zebrafish a model system in developmental biology. The existence of eyed surface (surface fish) and blind cave (cavefish) dwelling forms in Astyanax also provides an attractive system for studying the evolution of developmental mechanisms. The polarity of evolutionary changes and the environmental conditions leading to the cavefish phenotype are known with certainty, and several different cavefish populations have evolved constructive and regressive changes independently. The constructive changes include enhancement of the feeding apparatus (jaws, taste buds, and teeth) and the mechanosensory system of cranial neuromasts. The homeobox gene Prox 1, which is expressed in the expanded taste buds and cranial neuromasts, is one of the genes involved in the constructive changes in sensory organ development. The regressive changes include loss of pigmentation and eye degeneration. Although adult cavefish lack functional eyes, small eye primordia are formed during embryogenesis, which later arrest in development, degenerate, and sink into the orbit. Apoptosis and lens signaling to other eye parts, such as the cornea, iris, and retina, result in the arrest of eye development and ultimate optic degeneration. Accordingly, an eye with restored cornea, iris, and retinal photoreceptor cells is formed when a surface fish lens is transplanted into a cavefish optic cup, indicating that cavefish optic tissues have conserved the ability to respond to lens signaling. Genetic analysis indicates that multiple genes regulate eye degeneration, and molecular studies suggest that Pax6 may be one of the genes controlling cavefish eye degeneration. Further studies of the Astyanax system will contribute to our understanding of the evolution of developmental mechanisms in vertebrates.     

The role of the ocelli in the phototactic behaviour of the haematophagous bug Triatoma infestans

In addition to compound eyes, most adult insects posses two or three simple eyes, the ocelli. The function of these photoreceptors remains elusive in most cases. Triatomine bugs posses two well-developed ocelli, located in a latero-dorsal position, behind the compound eyes. We tested the role of the ocelli in the phototactic behaviour of Triatoma infestans, by measuring the time spent by adult males in the dark half of an experimental arena, which had the other half illuminated. The occlusion of the ocelli or the compound eyes alone had little effect on the phototactic response of the bugs. Only those insects which had both their ocelli and compound eyes occluded showed a significant reduction in their negative response to light. The ability of the ocelli of T. infestans to mediate the phototactic response by themselves (i.e., not through the modulation of compound eyes sensitivity) constitutes the first report on this function in insects.     

Regressive evolution: ontogeny and genetics of cavefish eye rudimentation

Two hypotheses exist to explain ontogenetic eye reduction in Astyanax cave fish: first, after lens induction by the primordial eye cup, the lens plays the role of a central regulator of eye and retina regression or, second, the retina itself is an independent unit of eye development. A comparative study of five blind cave fish populations and their surface sister form was performed to investigate the differences of ontogenetic eye regression between the cave populations during different stages of development. The study revealed that, in addition to the initial formation of smaller primordia, eye regression is also caused during later ontogeny by different relative growth and specific histological characteristics. Whereas the cave fish lens never properly differentiates, the regressive process of the retina is transitorily interrupted by ongoing differentiation. In the newly-discovered Molino cave population, even visual cells with well-organized outer segments develop, which are secondarily reduced at a later ontogenetic stage. This result shows that the retina and lens are independent developmental units within the eye ball. Presumably, the genetic systems responsible for both show independent inheritance, which is also corroborated by hybrids of F ₂-crosses between the cave and surface fish, in which lens and retina development do not correlate. During ontogeny, the eye size differs between the cave populations. In Pachón cave fish, the relatively large eye size correlates with an ancient introgression from a surface population, which may have delayed eye regression.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 287–296.     

The lens controls cell survival in the retina: Evidence from the blind cavefish Astyanax

The lens influences retinal growth and differentiation during vertebrate eye development but the mechanisms are not understood. The role of the lens in retinal growth and development was studied in the teleost Astyanax mexicanus, which has eyed surface-dwelling (surface fish) and blind cave-dwelling (cavefish) forms. A lens and laminated retina initially develop in cavefish embryos, but the lens dies by apoptosis. The cavefish retina is subsequently disorganized, apoptotic cells appear, the photoreceptor layer degenerates, and retinal growth is arrested. We show here by PCNA, BrdU, and TUNEL labeling that cell proliferation continues in the adult cavefish retina but the newly born cells are removed by apoptosis. Surface fish to cavefish lens transplantation, which restores retinal growth and rod cell differentiation, abolished apoptosis in the retina but not in the RPE. Surface fish lens deletion did not cause apoptosis in the surface fish retina or affect RPE differentiation. Neither lens transplantation in cavefish nor lens deletion in surface fish affected retinal cell proliferation. We conclude that the lens acts in concert with another optic component, possibly the RPE, to promote retinal cell survival. Accordingly, deficiency in both optic structures may lead to eye degeneration in cavefish.     

Neutrale Mutationen und evolutionäre Fortentwicklung

Neutral mutation and evolutionary progress The process and causes of regressive evolution are still under debate. Contrary to DARWIN'S original assumption, Neo-Darwinian proponents make selection responsible for reduction. Biologically functionless structures like eye and pigmentation in cave animals deliver excellent material to study this problem. Comparison of regressive (eye, pigmentation, aggression, dorsal light reaction) and constructive traits (gustatory equipment, egg yolk content, feeding behavior) in epigean and cave fish (Astyanax fasciatus, Characidae) reveal a high variability of the regressive features in the cave forms. Contrary to this, the constructive traits are characterized by a low variability in epigean and cave fish. This difference is attributed to the lack of selection on regressive structures. The existence of an intermediate cave population between epigean and true cave fish of A. fasciatus makes possible the study of evolutionary rates. It is shown that the regressive traits do not evolve more quickly than the constructive ones do. On the contrary, constructive traits like egg yolk content are even more rapid because they are of great biological value in the cave biotope. Especially energy economy is claimed by Neo-Darwinists to play a decisive role as a selective force. Comparison of the development of epi- and hypogean larvae of A. fasciatus shows that the formation of a smaller and less differentiated eye in the cave specimens has no effect on body growth. Furthermore, even behavioral traits like aggressiveness, schooling, dorsal light reaction, or negative phototaxis, which all are not performed in darkness by the epigean ancestor, become genetically reduced in the cave fish. The principles of regressive evolution, loss of selection and increase in variability, play a central role in evolution in general. When biota with empty niches are colonized, stabilizing selection relaxes from the special adaptations to the niche inhabited before by the invading species. Variability may arise in these and is permitted as long as fitness is guaranteed. Such processes characterize adaptive radiation. Examples are given by the species flocks on isolated islands or in chemically abnormal lakes like those of the East African Rift Valley. Only secondarily, on the basis of the arisen variability, does directional selection promote the newly developing species into different niches. The loss of stabilizing selection is an important factor for the evolutionary process to be open for evolutionary progress.     

Der Micos-Fisch,Höhlenfisch in statu nascendi oder Bastard ?

The Micos Population–cave fish in statu nascendi or hybrid? Observations on the evolution of cavernicoles The Micos-Cave in the Sierra de la Colmena in the State of San Luis Potosi, Mexico, contains a cavernicolous population of Astyanax mexicanus, whose members are for the most part blind, but, in contrast to other cave dwelling populations of the same species, appear almost normal in their pigmentation. Besides these, there are also large eyed and pigmented specimens to be found in this cave. Any transitional stages between the blind and the normal visioned fish are lacking. Offspring of the blind cavernicoles that are raised under light conditions develop a superficially lying eye which is markedly smaller than normal, attaining its size proportional to the light-intensity of the experimental conditions. The size and structure of the eye-remnants of the blind fish as well as the eyes of their offspring are considerably more variable than in the river specimens. Crossings of the blind Micos fish with the river fish Astyanax and also with a blind and unpigmented troglobiont of the same species - Sabinos fish - result in both cases in a more or less intermediate F¹-hybrid. A strict inbreeding within the Micos fish, selecting specimens with especially large eyes, produced animals whose eyes are comparable to those of the river fish after only three offspring generations. Electrophoresis studies on the allozyme variability at various loci prove that the Micos fish is genetically only slightly different compared to the river fish. On the other hand in some allele frequencies there is an alternative variation between the two. The Micos fish also differs from the typical troglobionts which are monomorphic at almost all loci examined and also possess alleles that are not found in the river form. Based on the genetic constitution, the Micos fish and the river fish found in the cave do not form a panmictic population. It is also doubtful that the Micos fish is the progeny of a hybrid swarm which previously resulted from a cross between a real troglobiont and the newly arrived river fish, because the Micos fish is in every characteristic genetically very similar to the river fish, whereas no clear traces of troglobiont relationship are found. Thus the Micos fish actually appears to be a cave form in statu nascendi against which the river fish that find their way in from time to time cannot compete.     

Phototactic response and light sensitivity in an epigean and a hypogean population of a barb (Garra barreimiae, Cyprinidae)

We investigated the response of two populations of the barb Garra barreimiae to different light intensities (0.5–2000 lx) from a light source. Adults of both the surface (epigean) and cave (hypogean) G. barreimiae populations show photophobic behavior. A photophobic response in the cave form was seen only at higher light intensities because the cavefish are eyeless and rely on extra-retinal light receptors to detect light. In contrast, juveniles (surface and cave) showed photophilic behavior, and their preference for the photic zone of the test tank decreased with increasing age. We discuss the potential role played by photophobic behavior for the colonization of caves by previously surface-dwelling fishes.     

Non-optical releasers for aggressive behavior in blind and blinded Astyanax (Teleostei, Characidae)

Prior to this study, it was believed that epigean and hypogean Astyanax differ markedly in their display of agonistic behavior. Research suggested that surface-dwelling individuals were extremely aggressive whereas their blind, cave-dwelling counterparts tended to show little or no aggressive behavior. Aggression in Astyanax was thought to be triggered by visual stimuli because surface fish in a dark environment or surface fish blinded late in life did not show aggression. Here, we demonstrate that surface fish blinded early on in their embryonic development are highly aggressive as adults. We also report the first case of a population of blind cave-dwelling Astyanax that is highly aggressive. We conclude that reduced aggression is not the only evolutionary pathway for troglobitic Astyanax and that there is some degree of developmental plasticity in the releaser of aggression and in the selection of its triggering stimuli.     

Development and evolution of craniofacial patterning is mediated by eye-dependent and -independent processes in the cavefish Astyanax

We studied the development and evolution of craniofacial features in the teleost fish, Astyanax mexicanus. This species has an eyed surface dwelling form (surface fish) and many different cave dwelling forms (cavefish) with various degrees of reduced eyes and pigmentation. The craniofacial features we examined are the tooth-bearing maxillary bones, the nasal and antorbital bones, the circumorbital bones, and the opercular bones, all of which show evolutionary modifications in different cavefish populations. Manipulations of eye formation by transplantation of the embryonic lens, by lentectomy, or by removing the optic vesicle showed that eye-dependent and -independent processes change both the surface fish and cavefish craniofacial skeletons. The size of the olfactory pits, which the nasal and antorbital bones define, and the size and positioning of the circumorbital bones were found to correlate with eye development. For the six suborbital bones (SO1-6), the relationship with the developing eye appears to be due to ossification initiated from foci in the suborbital canal of cranial neuromasts, whose patterning is also highly correlated with the presence or absence of an eye. By contrast, we found that the number of maxillary teeth, the number of SO3 bone elements, the positioning of SO4-6 with respect to the opercular bone, and the shape of the opercular bone are not dependent on eye formation and vary among different cavefish populations. The results suggest that evolution of the cavefish craniofacial skeleton is controlled by multiple developmental events, some a direct consequence of eye degeneration and others unrelated to loss of the eye.     

BY-F剑尾鱼白内障的观察

目的了解BY-F近交剑尾鱼白内障的发展及其对剑尾鱼生存的影响。方法观察眼球出现混浊的剑尾鱼,定期观察眼球病变的发展情况以及眼病引起的外部形态和行为的变化;对病鱼的眼球等进行组织病理观察。结果病鱼一般体色晦暗,眼球可见不同程度的圆环状浑浊,后期发展有角膜表面出现红色增生物等现象;组织病理观察发现,其主要病变在晶状体。结论所发现的剑尾鱼眼睛疾患为白内障;剑尾鱼的白内障后期发展可导致其他眼睛疾病并发症;BY-F剑尾鱼是白内障的易发群体。     

Aggressive behaviour of an epigean population of Astyanax mexicanus (Characidae,Pisces) and some observations of three subterranean populations

The different populations have been tested for aggressive behaviour in groups of four to eight animals of both sexes in tanks ranging from 251 to 9501. In darkness less aggressive behaviour has been observed with the help of an infrared video-camera in the epigean fish and the eyed Micos cave fish. The strongest degree of reduction in aggressive behaviour is shown by the totally blind populations. The aggressiveness increases in the epigean fish as soon as space and food supply diminish.