Genetics and hybridization in surface and cave Astyanax (Teleostei): a comparison of regressive and constructive traits

Change in ecological conditions, as seen in surface and cave populations of Astyanax (Teleostei), has caused the divergent evolution of a large number of traits like eyes, coloration, taste, lateral line, and different kinds of behaviour like schooling, sleep or feeding posture. Because of the interfertility of surface and cave forms these fish are an exceptional object to study the morphological and genetic basis of the evolution of such complex regressive and constructive traits. Classical crossing analyses and genomic studies are contributing to growing understanding. Both kinds of traits mostly rely on multiple genetic bases and the phenotypic manifestation in the various crosses is similar. The gene effect underlying the phenotypic manifestation may exhibit an exponential increase at differing amounts in the various traits and crosses. Missing or presence of such genetic interaction helps determine whether the variability of eyes or pigmentation exhibited by Astyanax cave fish populations like Micos, is due to a more recent origin or to secondary hybridization with the surface fish. Neither crossing analysis nor QTL mapping revealed that eye reduction is pleiotropically antagonistically related to the increase of taste buds or lateral line sense. Independent inheritance of traits suggests that Astyanax cave fish are subjected to mosaic evolution.     

Parallel speciation in Astyanax cave fish (Teleostei) in Northern Mexico

We investigated differentiation processes in the Neotropical fish Astyanax that represents a model system for examining adaptation to caves, including regressive evolution. In particular, we analyzed microsatellite and mitochondrial data of seven cave and seven surface populations from Mexico to test whether the evolution of the cave fish represents a case of parallel evolution. Our data revealed that Astyanax invaded northern Mexico across the Trans-Mexican Volcanic Belt at least three times and that populations of all three invasions adapted to subterranean habitats. Significant differentiation was found between the cave and surface populations. We did not observe gene flow between the strongly eye and pigment reduced old cave populations (Sabinos, Tinaja, Pachon) and the surface fish, even when syntopically occurring like in Yerbaniz cave. Little gene flow, if any, was found between cave populations, which are variable in eye and pigmentation (Micos, Chica, Caballo Moro caves), and surface fish. This suggests that the variability is due to their more recent origin rather than to hybridization. Finally, admixture of the young Chica cave fish population with nuclear markers from older cave fish demonstrates that gene flow between populations that independently colonized caves occurs. Thus, all criteria of parallel speciation are fulfilled. Moreover, the microsatellite data provide evidence that two co-occurring groups with small sunken eyes and externally visible eyes, respectively, differentiated within the partly lightened Caballo Moro karst window cave and might represent an example for incipient sympatric speciation.     

Regressive und rekonstruktive Evolution bei Höhlenfischen -zwei Formen besonders schnellen phylogenetischen Wandels

Regressive and reconstructive evolution in cave fishes - two modes of especially rapid phylogenetic change If a characteristic loses its biological significance, it is no IonGer reulated by a stabilizing selection rocess. Its polygene system will be progressively inactivatecf by continual mutation, and the characteristic will degenerate by regressive evolution. However, if a system of modifying genes faces uncontrolled exposure to general mutation pressure and gradually loses its effectiveness, a derepression of fundamental genetic potencies can occur, bringing about the expansion of a trait that is already present, a reconstructive evolution. Both modes of evolution result in the extensive morphological and behavioral alterations of cave fishes before a reproductive separation from their surface-dwelling ancestral forms has occurred, making them distinct species.     

Mapping a cave fish genome: polygenic systems and regressive evolution

We used random amplified polymorphic DNA (RAPD) fingerprinting to generate anonymous DNA markers in the fish Astyanax mexicanus, a species with both surface and cave populations. Surface individuals are eyed and pigmented; troglobitic forms are blind and depigmented. We hybridized surface fish and Pachon population cave fish and produced a RAPD genomic map 1064 cM in length (about half the total length of the genome) that was used to screen for quantitative trait loci (QTL) for troglomorphic traits. Three QTL for reduced eye size, two for decreased numbers of melanophores, two for condition factor, and the locus for the unifactorial trait, albinism, were mapped. These factors account for an average of 46% of the variance in these traits in the backcross. The results are the first direct demonstration that troglomorphic changes in this population are multifactorial. Two closely linked pairs of QTL were found. Each consisted of a regressive and a constructive trait QTL. These close linkages are unlikely to have occurred by chance (P <.05 for each) and suggest that troglomorphic evolution might be facilitated by pleiotropy or by genetic hitchhiking.     

A review of Morphological and Behavioural Changes in the Cave Molly, Poecilia Mexicana, from Tabasco, Mexico

The poeciliid fish Poecilia mexicana successfully colonised a sulfur cave in Tabasco, Mexico. The eye size, melanin pigmentation and scale tuberances as well as aggressive and schooling behaviour are reduced in the cave fish. Besides these regressive characters some constructive ones have been found. The genital region of the female is enlarged to a genital pad and in both sexes the number of taste buds is considerably increased on the upper side of the head. The cephalic lateral line shows a hyperdevelopment with an incomplete covering of some channels. Nearly all of these characters studied form a genetically based gradient from the beginning to the end of the cave, which seems to be the result of gene flow from the outside to the inner part of the cave. A special behavioural adaptation was found among females which prefer bigger males during courtship. In darkness, only the cave females show this preference.     

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.     

Accepting evolution

Poor public perceptions and understanding of evolution are not unique to the developed and more industrialized nations of the world. International resistance to the science of evolutionary biology appears to be driven by both proponents of intelligent design and perceived incompatibilities between evolution and a diversity of religious faiths. We assessed the success of a first-year evolution course at the University of Cape Town and discovered no statistically significant change in the views of students before the evolution course and thereafter, for questions that challenged religious ideologies about creation, biodiversity, and intelligent design. Given that students only appreciably changed their views when presented with "facts," we suggest that teaching approaches that focus on providing examples of experimental evolutionary studies, and a strong emphasis on the scientific method of inquiry, are likely to achieve greater success. This study also reiterates the importance of engaging with students' prior conceptions, and makes suggestions for improving an understanding and appreciation of evolutionary biology in countries such as South Africa with an inadequate secondary science education system, and a dire lack of public engagement with issues in science.     

EVIDENCE FOR REPEATED LOSS OF SELECTIVE CONSTRAINT IN RHODOPSIN OF AMBLYOPSID CAVEFISHES (TELEOSTEI: AMBLYOPSIDAE)

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.     

One eye but no vision: cave fish with induced eyes do not respond to light

One of the most intriguing questions in evolutionary biology is the degree to which behavior is a necessary consequence of morphology. We explore this issue by examining phototactic behavior in epigean (eyed surface-dwelling) and troglomorphic (blind cave) forms of the teleost Astyanax fasciatus whose eyes were modified during embryogenesis by removing one or both lens vesicles from the epigean form or by transplanting the lens vesicle from an epigean fish into the optic cup of a blind cave form. Lens removal results in eye degeneration and blindness in adult epigean fish, whereas lens transplantation stimulates growth of the eye, inducing the development of optic tissues in the normally eyeless adult cave fish. Photoresponsiveness was examined by placing fish in an aquarium with one half illuminated and the other half dark and scoring their presence in the illuminated or dark half. Both the eyeless epigean fish and cave fish with induced eyes are indifferent to the illumination whereas the surface forms are scotophilic, suggesting that optic development and phototactic behavior are decoupled.     

Comparative ultrastructural investigations of the pineal organ of the blind cave fish,Anoptichthys jordani,and its ancestor,the eyed river fish,Astyanax mexicanus

A comparative ultrastructural study has been made of the pineal organ in specimens of two closely related populations of the characid fish, Astyanaz mexicanus. The specimens of one population are living in the river, under natural light conditions. The specimens of the other population, originally described as Anoptichthys jordani, are living in a completely dark cave. In specimens of both populations the pineal organ consists of a spindle shaped end-vesicle, connected to the diencephalic roof by a slender stalk. The pineal tissue is compact and consists predominantly of glia-like supporting cells and sensory cells resembling the photoreceptor cells of the lateral vertebrate eye. Phagocytotic microglia-like cells can be found in close contact with the outer segments of the sensory cells. Nerve cells are located in the neighbourhood of neuropil formations, in which synaptic contacts are established between sensory cells and nerve cells. From these nerve cells fibers are emerging, forming the pineal tract that runs down the pineal stalk towards the diencephalon. On the basis of the ultrastructure described by other authors it is concluded that the pineal organ in specimens of the river population of Astyanax mexicanus resembles the pineal organ of other fish species. In specimens of the river population, reared under normal light-dark conditions for 3, 9 or 18 months, conspicuous morphological changes have not been detected in the presumably light-sensitive outer segments of the sensory cells or in other parts of the pineal tissue. In specimens of the cave populations, reared under identical conditions, an age-dependent, gradual regression of the regular outer segment organization of the pineal sensory cells takes place. In other parts of the pineal tissue, only small morphological changes can be observed. In specimens of the cave population, reared in constant darkness, the regression of the pineal outer segment organization begins earlier and is obvious. It is postulated that the gradual age-dependent regression of the regular organization of the outer segments in the pineal organ of cave specimens of Astyanax mexicanus is genetically determined and indicates a regressive evolution of the pineal light sensitivity. The expression of the regressive traits is dependent on the environmental light conditions.     

Spatial parameters encoded in the spatial map of the blind Mexican cave fish, Astyanax fasciatus

Although much is now known about the mechanisms that insects, birds and mammals use to orient within familiar areas, our knowledge of such mechanisms in fish is scant. I used the transformational approach to test whether the blind Mexican cave fish can encode shape and size in an internal representation of space. These fish are excellent study animals, as they swim at high velocities (presumably to enhance lateral line organ stimulation) when faced with unfamiliar landmarks or environments. As they are blind, potentially confounding cues from visual global landmarks are unavailable. The fish learnt a square configuration of four landmarks and so must have been be able to encode spatial relationships between the elements within this configuration. After learning landmark arrays, the cave fish showed significant dishabituation (swimming velocity was increased) when exposed to landmark transformations. The fish must therefore have been comparing the environment that they perceived with an internal representation of the environment that they had learnt. The results show that blind Mexican cave fish can encode size (absolute distance between landmarks) and possibly also shape within their spatial maps.     

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.     

A Novel Role for Mc1r in the Parallel Evolution of Depigmentation in Independent Populations of the Cavefish Astyanax mexicanus

The evolution of degenerate characteristics remains a poorly understood phenomenon. Only recently has the identification of mutations underlying regressive phenotypes become accessible through the use of genetic analyses. Focusing on the Mexican cave tetra Astyanax mexicanus, we describe, here, an analysis of the brown mutation, which was first described in the literature nearly 40 years ago. This phenotype causes reduced melanin content, decreased melanophore number, and brownish eyes in convergent cave forms of A. mexicanus. Crosses demonstrate non-complementation of the brown phenotype in F₂ individuals derived from two independent cave populations: Pachón and the linked Yerbaniz and Japonés caves, indicating the same locus is responsible for reduced pigmentation in these fish. While the brown mutant phenotype arose prior to the fixation of albinism in Pachón cave individuals, it is unclear whether the brown mutation arose before or after the fixation of albinism in the linked Yerbaniz/Japonés caves. Using a QTL approach combined with sequence and functional analyses, we have discovered that two distinct genetic alterations in the coding sequence of the gene Mc1r cause reduced pigmentation associated with the brown mutant phenotype in these caves. Our analysis identifies a novel role for Mc1r in the evolution of degenerative phenotypes in blind Mexican cavefish. Further, the brown phenotype has arisen independently in geographically separate caves, mediated through different mutations of the same gene. This example of parallelism indicates that certain genes are frequent targets of mutation in the repeated evolution of regressive phenotypes in cave-adapted species.     

Potential rapid evolution of foot morphology in Italian plethodontid salamanders (Hydromantes strinatii) following the colonization of an artificial cave

How organisms respond to environmental change is a long‐standing question in evolutionary biology. Species invading novel habitats provide an opportunity to examine contemporary evolution in action and decipher the pace of evolutionary change over short timescales. Here, we characterized phenotypic evolution in the Italian plethodontid salamander, Hydromantes strinatii, following the recent colonization of an artificial cave by a forest floor population. When compared with a nearby and genetically related population in the natural forest floor and a nearby cave population, the artificial cave population displayed significant differences in overall foot shape, with more interdigital webbing relative to the other populations. Further, this population evolved significantly larger feet, which corresponded more closely to those found in other cave populations than to forest floor populations to which the cave population is closely related. Finally, we quantified the rate of evolution for both foot shape and foot area, and found that both traits displayed large and significant evolutionary rates, at levels corresponding to other classic cases of rapid evolution in vertebrates. Together, these findings reveal that the response to novel environmental pressures can be large and rapid and that the anatomical shifts observed in the artificial cave population of H. strinatii may represent a case of rapid evolution in response to novel environmental pressures.     

哺乳动物毛色调控机制及其适应性进化研究进展

哺乳动物类群呈现出的丰富毛色是引人注目的一种生物现象,是研究和理解哺乳动物适应性进化的理想模型之一。哺乳动物的毛色多态在躲避天敌、捕食、求偶及抵御紫外线等方面都具有重要作用。哺乳动物毛发的色素化过程由体内黑色素的数量、质量和分布状况所决定。黑色素的形成过程复杂,包括黑素细胞的分化、成熟,黑素体等细胞器的形态发生及黑色素在黑素细胞中的合成代谢和转运等过程;而在细胞色素化的每个阶段/时相都伴随着一些重要功能基因的参与,并通过基因之间的相互作用形成了黑色素生物代谢的复杂调控网络,进而形成不同的毛色有助于哺乳动物适应不同生存环境。对哺乳动物不同毛色形成机制的探究一直以来都是遗传学及进化生物学的重要研究领域和聚焦热点。本文综述了哺乳动物毛色色素化过程的主要分子机制以及毛色适应性进化的遗传基础,以期为哺乳动物毛色多态及其适应性进化的分子机制研究提供参考。     

洞穴鱼类眼部退化机制的研究进展

黑暗环境中的洞穴鱼类眼部结构发生了退化,但不同洞穴鱼物种眼部退化程度存在差异,从眼部结构的部分缺失到完全消失的情况均存在。目前研究表明,不论是达尔文的自然选择学说,还是木村资生的中性进化理论均不能很好地解释洞穴鱼类眼部退化的产生机制。洞穴鱼类眼部退化是一个复杂的过程,若要揭示其机制需汇集多个学科的研究优势。该文介绍了国内外洞穴鱼类眼部退化研究领域的形态解剖学、发育生物学、动物行为学及分子遗传学研究进展,并对洞穴鱼眼部退化的研究现状与发展提出了一些思考和建议。     

The emotion system promotes diversity and evolvability

Studies on the relationship between the optimal phenotype and its environment have had limited focus on genotype-to-phenotype pathways and their evolutionary consequences. Here, we study how multi-layered trait architecture and its associated constraints prescribe diversity. Using an idealized model of the emotion system in fish, we find that trait architecture yields genetic and phenotypic diversity even in absence of frequency-dependent selection or environmental variation. That is, for a given environment, phenotype frequency distributions are predictable while gene pools are not. The conservation of phenotypic traits among these genetically different populations is due to the multi-layered trait architecture, in which one adaptation at a higher architectural level can be achieved by several different adaptations at a lower level. Our results emphasize the role of convergent evolution and the organismal level of selection. While trait architecture makes individuals more constrained than what has been assumed in optimization theory, the resulting populations are genetically more diverse and adaptable. The emotion system in animals may thus have evolved by natural selection because it simultaneously enhances three important functions, the behavioural robustness of individuals, the evolvability of gene pools and the rate of evolutionary innovation at several architectural levels.     

Does rapid evolution matter? Measuring the rate of contemporary evolution and its impacts on ecological dynamics

Rapid contemporary evolution due to natural selection is common in the wild, but it remains uncertain whether its effects are an essential component of community and ecosystem structure and function. Previously we showed how to partition change in a population, community or ecosystem property into contributions from environmental and trait change, when trait change is entirely caused by evolution (Hairston et al. 2005). However, when substantial non-heritable trait change occurs (e.g. due to phenotypic plasticity or change in population structure) that approach can mis-estimate both contributions. Here, we demonstrate how to disentangle ecological impacts of evolution vs. non-heritable trait change by combining our previous approach with the Price Equation. This yields a three-way partitioning into effects of evolution, non-heritable phenotypic change and environment. We extend the approach to cases where ecological consequences of trait change are mediated through interspecific interactions. We analyse empirical examples involving fish, birds and zooplankton, finding that the proportional contribution of rapid evolution varies widely (even among different ecological properties affected by the same trait), and that rapid evolution can be important when it acts to oppose and mitigate phenotypic effects of environmental change. Paradoxically, rapid evolution may be most important when it is least evident.     

Quantitative Aspekte der regressiven Evolution – dargestellt am Beispiel der Augenrückbildung bei Höhlenfischen

Quantitative aspects of regressive evolution – demonstrated by the reduction of the eyes in cave fishes According to our present understanding, the eyes of cave fishes are no longer subject to a stabilizing selection. Because of a strong mutation pressure in the destructive direction, this leads to a continual enrichment of elimination mutants and thereby to a progressive reduction of the eyes after separation of a cave population. Obviously a natural selection process does not play a significant role in the reduction of the eyes in cave fishes. Based on these findings, the degeneration process can be characterized in detail, especially concerning its course and duration.     

A partial flip classroom exercise in a large introductory general biology course increases performance at multiple levels

Incorporation of active learning into large lecture classes is gaining popularity as a pedagogical method due to its known benefits in helping learning outcomes. A more recent active learning technique that has emerged is the flipped classroom. In this study, we investigated the effects of incorporating a ‘partial-flip’ into an introductory general biology course, where only a portion of the class time was spent in a flipped classroom format. This partial flip presented the classic biology experiment of Meselson and Stahl that discovered the mechanism of DNA replication. Performance on in-class formative assessments and subsequent summative assessments (eg out-of-class assignments and exams relating to this material) was compared between a class that had the partial flip and a control class that had a traditional lecture. The partial flip students scored higher on in-class formative questions, specific exam questions and final exam essay questions. We found that the partial flip had different effects in males versus females depending on the assessment. The partial flip manipulation appeared equally effective in aiding both below and above average students in formative assessments. Overall, this study shows that the partial flip classroom can be an effective technique to incorporate into existing courses and that it does provide some benefits compared to traditional lecture.