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Major histocompatibility complex (MHC) molecules are only found in jawed vertebrates and not in more primitive species. MHC class II type structures likely represent the ancestral structure of MHC molecules. Efficient MHC class II transport to endosomal compartments depends on association with a specialized chaperone, the MHC class II invariant chain (aliases Ii or CD74). The present study identifies conserved motifs in the CLIP region of CD74 molecules, used for binding in the MHC class II binding groove, throughout jawed vertebrates. Peculiarly, in CD74a molecules of Ostariophysi, a fish clade including for example Mexican tetra and zebrafish, the CLIP region has duplicated. In mammals, in endosomal compartments, the peptide-free form of classical MHC class II is stabilized by binding to nonclassical MHC class II "DM," a process that participates in "peptide editing" (selection for high affinity peptides). Hitherto, DM-lineage genes had only been reported from the level of amphibians, but the present study reveals the existence of DMA and DMB genes in lungfish. This is the first study which details how classical and DM lineage molecules have distinguishing glycine-rich motifs in their transmembrane regions. In addition, based on extant MHC class II structures and functions, the present study proposes a model for early MHC evolution, in which, in an ancestral jawed vertebrate, the ancestral MHC molecule derived from a heavy-chain-only antibody type molecule that cycled between the cell surface and endosomal compartments. [ABSTRACT FROM AUTHOR]

The lateral line system and its innervation were examined in the blind cavefish Astyanax mexicanus (Characidae) and compared with that in the eyed surface-dwelling form of the same species. In both forms, superficial neuromasts occurred on almost the entire surface of the body and caudal fin, the number in the blind form being greater on the head (831) and tail (511) than in the eyed form (531 and 251, respectively), but lower on the trunk (1,305 vs. 2,125). In the blind form, superficial neuromasts were greatly proliferated on the cheek and lower jaw, the mandibular ramus being extensively ramified, including an additional major ramule that was absent in the eyed form. In both forms, superficial neuromasts on the trunk were innervated by a number of dorsal and ventral ramules (roughly corresponding to the number of lateral line scales) derived from the lateral ramus. In both forms, canal neuromasts in the infraorbital and trunk lateral line canals were innervated by nerves without associated elements innervating superficial neuromasts. Accordingly, the lateral line scales each accommodated only a single canal neuromast. This lateral line scale condition was recognized as common in Characidae, being in sharp contrast to the condition in Cyprinidae (both included in Otophysa) in which the lateral line scales each had a vertical row of superficial neuromasts. [ABSTRACT FROM AUTHOR]

It was shown for the first time that the perception of the blind Mexican cavefish Astyanax fasciatus of the odour of food and targeted food search in response to it is completely controlled by olfaction. Blockage of olfactory reception deprives the fish of this ability. The odour concentration threshold (water extract of chironomid larvae) is 10 g/L. It is suggested that degeneration of visual organs does not intensify olfaction in troglobiont cavefish, but can cause its insignificant correction. Chronic anosmia (4 months) leads to weak recovery of chemoorientation, which supports the possession of external taste reception in blind cavefish. [ABSTRACT FROM AUTHOR]

The Mexican tetra, Astyanax mexicanus, comprises 29 populations of cave-adapted fish distributed across a vast karst region in northeastern Mexico. These populations have a complex evolutionary history, having descended from 'old' and 'young' ancestral surface-dwelling stocks that invaded the region ∼6.7 and ∼2.8 MYa, respectively. This study investigates a set of captive, pigmented Astyanax cavefish collected from the Micos cave locality in 1970, in which albinism appeared over the past two decades. We combined novel coloration analyses, coding sequence comparisons and mRNA expression level studies to investigate the origin of albinism in captive-bred Micos cavefish. We discovered that albino Micos cavefish harbor two copies of a loss-of-function ocular and cutaneous albinism type II (Oca2) allele previously identified in the geographically distant Pachón cave population. This result suggests that phylogenetically young Micos cavefish and phylogenetically old Pachón cave fish inherited this Oca2 allele from the ancestral surface-dwelling taxon. This likely resulted from the presence of the loss-of-function Oca2 haplotype in the 'young' ancestral surface-dwelling stock that colonized the Micos cave and also introgressed into the ancient Pachón cave population. The appearance of albinism in captive Micos cavefish, caused by the same loss-of-function allele present in Pachón cavefish, implies that geographically and phylogenetically distinct cave populations can evolve the same troglomorphic phenotype from standing genetic variation present in the ancestral taxon. [ABSTRACT FROM AUTHOR]

The extreme sensory environment of subterranean caves presents an adaptive challenge for troglobitic organisms. Although caves provide distinct selective pressures (e.g., darkness, food scarcity), advantages often come with living in isolated places. The cave-adapted (hypogean) form of the teleost Astyanax mexicanus lacks predators in its natural environment, unlike its ancestral surface (epigean) form that inhabits the nearby rivers. This species provides a unique opportunity for comparative studies because A. mexicanus has invaded the cave environment multiple times, providing us with independent comparative adaptive experiments. Here, we examine kinematic parameters that characterize the escape responses of various forms of A. mexicanus. We demonstrate that populations differ in details of fast start performance, but that surface fish do not perform better than cavefish in escape responses. Our results are more consistent with genetic drift or divergent selection among caves than with relaxation of selection in determining the evolution of escape behavior in this system. [ABSTRACT FROM AUTHOR]

The extreme environment of subterranean caves presents an adaptive challenge to troglobitic organisms. The mechanisms by which natural selection modify an ancestral surface neural circuit to produce a novel subterranean behavior remain a mystery. To address this question, we performed cross species microarray experiments to compare differences in gene expression levels in the adult brain of the teleost Astyanax mexicanus. This species provides a unique opportunity for comparative genetic studies as it consists of extant epigean (surface) and hypogean (cave) conspecifics. Microarray experiments herein revealed significant changes in transcription levels of seventeen genes, several of which are important for behaviors involved in metabolic management. We focused on genes central to three neurotransmission and neuromodulation networks: the endocannabinoid system (Cannabinoid receptor CB1), the dopaminergic system (Tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein) and the glutamatergic system (glutamate receptor AMPA 2a). All three genes were upregulated in the hypogean form of A. mexicanus compared to the epigean form, indicating that behavioral differences in the hypogean form of the species could be due to alterations in expression levels of several key genes. This information provides insights into the complex relationships among environmental factors, genetics, nervous systems and adaptive behavior, and can subsequently help us understand how these interactions affect behavior in other biological systems. [ABSTRACT FROM AUTHOR]

The article discusses the potential for new model organisms to emerge within the research realm of developmental biology. Clinical research has traditionally been conducted on a few specific species, including the mouse (Mus musculus), the fruitfly (Drosophila melanogaster), and the zebrafish (Danio rerio). There are inherent difficulties in straying from these established models, including the need for new genetic and genomic tools. Nonetheless, other organisms such as Notothenioidei (ice fish) are being considered as model organisms. Among this group of fish are the three-spined stickleback (Gasterosteus aculeatus) and the Mexican cave fish (Astyanax mexicanus).

We have conducted a survey of the expression patterns of five genes encoding three different classes of major lens proteins during eye degeneration in the blind cavefish Astyanax mexicanus. This species consists of two forms, an eyed surface-dwelling form (surface fish) and a blind cave-dwelling (cavefish) form. Cavefish form an optic primordium with a lens vesicle and optic cup. In contrast to surface fish, however, the cavefish lens does not differentiate fiber cells and undergoes massive apoptosis. The genes encoding the lens intrinsic membrane proteins MIP and MP19 and the divergent βB1- and γM2-crystallins are expressed during cavefish lens development, although their levels are reduced because of a smaller lens, and the spatial distribution of their transcripts is modified because of the lack of differentiated fiber cells. In contrast, the αA-crystallin gene, which encodes a heat shock protein-related chaperone with antiapoptotic activity, is substantially downregulated in the developing cavefish lens. The results suggest that suppression of αA-crystallin antiapoptotic activity may be involved in cavefish eye degeneration. [ABSTRACT FROM AUTHOR]

Characterization, management and protection of biodiversity are the most prominent challenges in conservation biology. Analyses on molecular similarity (Random Amplified Polymorphic DNA-Polymerase Chain Reaction, RAPD-PCR) and morphologic patterns (morphometric and meristic characters) were performed to evaluate the existence of population structuring in the Doce river basin, in a widespread small-sized characin, Astyanax bimaculatus. Three hundred (300) individuals were collected from six locations isolated by waterfalls or dams, in the Doce river basin, Minas Gerais State, Brazil. Genetic differentiation among tributaries was significant ( p<0.00001) and accounted for 21% of total variance ( p<0.00001). Only the Grande waterfall of the Casca river was an effective factor for differentiation of populations ( p<0.00001), and meristic characters were also consistent with molecular data. We concluded that genetic and morphologic variation of this species was not necessarily associated with waterfalls and that molecular and meristic data are effective predictors of population divergence in this basin. The consequences of these findings for the protection and management of aquatic biodiversity were discussed. [ABSTRACT FROM AUTHOR]

Hedgehog (Hh) proteins are responsible for critical signalling events during development but their evolutionary roles remain to be determined. Here we show that hh gene expression at the embryonic midline controls eye degeneration in blind cavefish. We use the teleost Astyanax mexicanus, a single species with an eyed surface-dwelling form (surface fish) and many blind cave forms (cavefish), to study the evolution of eye degeneration. Small eye primordia are formed during cavefish embryogenesis, which later arrest in development, degenerate and sink into the orbits. Eye degeneration is caused by apoptosis of the embryonic lens, and transplanting a surface fish embryonic lens into a cavefish optic cup can restore a complete eye. Here we show that sonic hedgehog (shh) and tiggy-winkle hedgehog (twhh) gene expression is expanded along the anterior embryonic midline in several different cavefish populations. The expansion of hh signalling results in hyperactivation of downstream genes, lens apoptosis and arrested eye growth and development. These features can be mimicked in surface fish by twhh and/or shh overexpression, supporting the role of hh signalling in the evolution of cavefish eye regression. [ABSTRACT FROM AUTHOR]

We have investigated expression of the homeobox gene Prox 1 during eye degeneration and sensory organ compensation in cavefish embryos. The teleost Astyanax mexicanus consists of sighted surface-dwelling forms (surface fish) and several populations of blind cave-dwelling forms (cavefish), which have evolved independently. Eye formation is initiated during cavefish development, but the lens vesicle undergoes apoptosis, and the eye subsequently arrests and degenerates. The requirement of Prox 1 for lens fiber differentiation and γ-crystallin expression in the mouse suggests that changes in the expression of this gene could be involved in cavefish eye degeneration. Surface fish and cavefish embryos stained with a Prox 1 antibody showed Prox 1 expression in the lens, neuroretina, myotomes, heart, hindbrain, and gut, as reported in other vertebrates. We found that Prox 1 expression is not altered during cavefish lens development. Prox 1 protein was detected in the lens vesicle as soon as it formed and persisted until the time of lens degeneration in each cavefish population. The cavefish lens vesicle was also shown to express a γ-crystallin gene, suggesting that Prox 1 is functional in cavefish lens development. In addition to the tissues described above, Prox 1 is expressed in developing taste buds and neuromasts in cavefish, which are enhanced to compensate for blindness. It is concluded that the Prox 1 gene is not involved in lens degeneration, but that expansion of the Prox 1 expression domain occurs during taste bud and neuromast development in cavefish. [ABSTRACT FROM AUTHOR]

A comparative ultrastructural study has been made of the pineal organ in specimens of two closely related populations of the characid fish, Astyanax 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. [ABSTRACT FROM AUTHOR]