Genetic identification and reiterated captures suggest that the Astyanax mexicanus El Pachon cavefish population is closed and declining.
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ABSTRACT:
SUBMITTER: Legendre L
PROVIDER: S-EPMC10415772 | biostudies-literature | 2023 Jul
REPOSITORIES: biostudies-literature
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Genetic identification and reiterated captures suggest that the <i>Astyanax mexicanus</i> El Pachón cavefish population is closed and declining.
Legendre Laurent L Rode Julie J Germon Isabelle I Pavie Marie M Quiviger Carla C Policarpo Maxime M Leclercq Julien J Père Stéphane S Fumey Julien J Hyacinthe Carole C Ornelas-García Patricia P Espinasa Luis L Rétaux Sylvie S Casane Didier D
Zoological research 20230701 4
The sizes of <i>Astyanax mexicanus</i> blind cavefish populations of North-East Mexico are demographic parameters of great importance for investigating a variety of ecological, evolutionary, and conservation issues. However, few estimates have been obtained. For these mobile animals living in an environment difficult to explore as a whole, methods based on capture-mark-recapture are appropriate, but their feasibility and interpretation of results depend on several assumptions that must be carefu ...[more]
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Project description:Distinct populations of Astyanax mexicanus cavefish offer striking examples of repeatable convergence or parallelism in their independent evolutions from surface to cave phenotypes. However, the extent to which the repeatability of evolution occurred at the genetic level remains poorly understood. To address this, we first characterized the genetic diversity of 518 single-nucleotide polymorphisms (SNPs), obtained through RAD tag sequencing and distributed throughout the genome, in seven cave and three groups of surface populations. The cave populations represented two distinct lineages (old and new). Thirty-one SNPs were significantly differentiated between surface and old cave populations, two SNPs were differentiated between surface and new cave populations, and 44 SNPs were significantly differentiated in both old and new cave populations. In addition, we determined whether these SNPs map to the same locations of previously described quantitative trait loci (QTL) between surface and cave populations. A total of 25 differentiated SNPs co-map with several QTL, such as one containing a fibroblast growth factor gene (Fgf8) involved in eye development and lens size. Further, the identity of many SNPs that co-mapped with QTL was the same in independently derived cave populations. These conclusions were further confirmed by haplotype analyses of SNPs within QTL regions. Our findings indicate that the repeatability of evolution at the genetic level is substantial, suggesting that ancestral standing genetic variation significantly contributed to the population genetic variability used in adaptation to the cave environment.
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Project description:BackgroundCave animals converge evolutionarily on a suite of troglomorphic traits, the best known of which are eyelessness and depigmentation. We studied 11 cave and 10 surface populations of Astyanax mexicanus in order to better understand the evolutionary origins of the cave forms, the basic genetic structuring of both cave and surface populations, and the degree to which present day migration among them affects their genetic divergence.ResultsTo assess the genetic structure within populations and the relationships among them we genotyped individuals at 26 microsatellite loci. We found that surface populations are similar to one another, despite their relatively large geographic separation, whereas the cave populations are better differentiated. The cave populations we studied span the full range of the cave forms in three separate geographic regions and have at least five separate evolutionary origins. Cave populations had lower genetic diversity than surface populations, correlated with their smaller effective population sizes, probably the result of food and space limitations. Some of the cave populations receive migrants from the surface and exchange migrants with one another, especially when geographically close. This admixture results in significant heterozygote deficiencies at numerous loci due to Wahlund effects. Cave populations receiving migrants from the surface contain small numbers of individuals that are intermediate in both phenotype and genotype, affirming at least limited gene flow from the surface.ConclusionsCave populations of this species are derived from two different surface stocks denoted "old" and "new." The old stock colonized caves at least three times independently while the new stock colonized caves at least twice independently. Thus, the similar cave phenotypes found in these caves are the result of repeated convergences. These phenotypic convergences have occurred in spite of gene flow from surface populations suggesting either strong natural or sexual selection for alleles responsible for the cave phenotype in the cave environment.
| S-EPMC3282648 | biostudies-literature
Project description:BackgroundCavefish populations belonging to the Mexican tetra species Astyanax mexicanus are outstanding models to study the tempo and mode of adaptation to a radical environmental change. They are currently assigned to two main groups, the so-called "old" and "new" lineages, which would have populated several caves independently and at different times. However, we do not have yet accurate estimations of the time frames of evolution of these populations.ResultsWe reanalyzed the geographic distribution of mitochondrial and nuclear DNA polymorphisms and we found that these data do not support the existence of two cavefish lineages. Using IMa2, a program that allows dating population divergence in addition to demographic parameters, we found that microsatellite polymorphism strongly supports a very recent origin of cave populations (< 20,000 years). We identified a large number of single-nucleotide polymorphisms (SNPs) in transcript sequences of pools of embryos (Pool-seq) belonging to Pachón cave population and a surface population from Texas. Based on summary statistics that can be computed with this SNP data set together with simulations of evolution of SNP polymorphisms in two recently isolated populations, we looked for sets of demographic parameters that allow the computation of summary statistics with simulated populations that are similar to the ones with the sampled populations. In most simulations for which we could find a good fit between the summary statistics of observed and simulated data, the best fit occurred when the divergence between simulated populations was less than 30,000 years.ConclusionsAlthough it is often assumed that some cave populations have a very ancient origin, a recent origin of these populations is strongly supported by our analyses of independent sets of nuclear DNA polymorphism. Moreover, the observation of two divergent haplogroups of mitochondrial and nuclear genes with different geographic distributions support a recent admixture of two divergent surface populations, before the isolation of cave populations. If cave populations are indeed only several thousand years old, many phenotypic changes observed in cavefish would thus have mainly involved the fixation of genetic variants present in surface fish populations and within a very short period of time.
| S-EPMC5905186 | biostudies-literature
Project description:Astyanax mexicanus, a teleost fish that exists in a river-dwelling surface form and multiple cave-dwelling forms, is an excellent system for studying the genetic basis of evolution. Cavefish populations, which independently evolved from surface fish ancestors multiple times, have evolved a number of morphological and behavioral traits. Quantitative trait loci (QTL) analyses have been performed to identify the genetic basis of many of these traits. These studies, combined with recent sequencing of the genome, provide a unique opportunity to identify candidate genes for these cave-specific traits. However, tools to test the requirement of these genes must be established to evaluate the role of candidate genes in generating cave-specific traits. To address this need, we designed transcription activator-like effector nucleases (TALENs) to target two genes that contain coding changes in cavefish relative to surface fish and map to the same location as QTL for pigmentation, oculocutaneous albinism 2 (oca2) and melanocortin 1 receptor (mc1r). We found that surface fish genes can be mutated using this method. TALEN-induced mutations in oca2 result in mosaic loss of melanin pigmentation visible as albino patches in F0 founder fish, suggesting biallelic gene mutations in F0s and allowing us to evaluate the role of this gene in pigmentation. The pigment cells in the albino patches can produce melanin upon treatment with L-DOPA, behaving similarly to pigment cells in albino cavefish and providing additional evidence that oca2 is the gene within the QTL responsible for albinism in cavefish. This technology has the potential to introduce a powerful tool for studying the role of candidate genes responsible for the evolution of cavefish traits.
| S-EPMC4361574 | biostudies-literature
Project description:We found higher substitution rates in cavefish compared with surface fish, in accordance with a smaller cavefish population size which has allowed more rapid fixation of derived alleles present in the ancestral population. This result also implies that the Pachn cave population is much younger than previously estimated. The comparison of these data with simulations suggests that the Pachn cavefish population has probably been underground less than 30,000 years. This new time frame, together with other evidence, indicate that the evolution of cave phenotypes mainly involves the fixation of cryptic genetic variants present in surface fish populations within a short period of time.
2016-06-27 | E-MTAB-5142 | biostudies-arrayexpress
Project description:The retina is the light-sensitive tissue of the eye that facilitates vision. Mutations within genes affecting eye development and retinal function cause a host of degenerative visual diseases, including retinitis pigmentosa and anophthalmia/microphthalmia. The characin fish Astyanax mexicanus includes both eyed (surface fish) and eyeless (cavefish) morphs that initially develop eyes with normal retina; however, early in development, the eyes of cavefish degenerate. Since both surface and cave morphs are members of the same species, they serve as excellent evolutionary mutant models with which to identify genes causing retinal degeneration. In this study, we crossed the eyed and eyeless forms of A. mexicanus and quantified the thickness of individual retinal layers among 115 F(2) hybrid progeny. We used next generation sequencing (RAD-seq) and microsatellite mapping to construct a dense genetic map of the Astyanax genome, scan for quantitative trait loci (QTL) affecting retinal thickness, and identify candidate genes within these QTL regions. The map we constructed for Astyanax includes nearly 700 markers assembled into 25 linkage groups. Based on our scans with this map, we identified four QTL, one each associated with the thickness of the ganglion, inner nuclear, outer plexiform, and outer nuclear layers of the retina. For all but one QTL, cavefish alleles resulted in a clear reduction in the thickness of the affected layer. Comparative mapping of genetic markers within each QTL revealed that each QTL corresponds to an approximately 35 Mb region of the zebrafish genome. Within each region, we identified several candidate genes associated with the function of each affected retinal layer. Our study is the first to examine Astyanax retinal degeneration in the context of QTL mapping. The regions we identify serve as a starting point for future studies on the genetics of retinal degeneration and eye disease using the evolutionary mutant model Astyanax.
| S-EPMC3577753 | biostudies-literature
Project description:The ability of organisms to adapt to sudden extreme environmental changes produces some of the most drastic examples of rapid phenotypic evolution. The Mexican Tetra, Astyanax mexicanus, is abundant in the surface waters of northeastern Mexico, but repeated colonizations of cave environments have resulted in the independent evolution of troglomorphic phenotypes in several populations. Here, we present three chromosome-scale assemblies of this species, for one surface and two cave populations, enabling the first whole-genome comparisons between independently evolved cave populations to evaluate the genetic basis for the evolution of adaptation to the cave environment. Our assemblies represent the highest quality of sequence completeness with predicted protein-coding and noncoding gene metrics far surpassing prior resources and, to our knowledge, all long-read assembled teleost genomes, including zebrafish. Whole-genome synteny alignments show highly conserved gene order among cave forms in contrast to a higher number of chromosomal rearrangements when compared with other phylogenetically close or distant teleost species. By phylogenetically assessing gene orthology across distant branches of amniotes, we discover gene orthogroups unique to A. mexicanus. When compared with a representative surface fish genome, we find a rich amount of structural sequence diversity, defined here as the number and size of insertions and deletions as well as expanding and contracting repeats across cave forms. These new more complete genomic resources ensure higher trait resolution for comparative, functional, developmental, and genetic studies of drastic trait differences within a species.
| S-EPMC11304944 | biostudies-literature
Project description:The ability of organisms to adapt to sudden extreme environmental changes produces some of the most drastic examples of rapid phenotypic evolution. The Mexican Tetra, Astyanax mexicanus, is abundant in the surface waters of northeastern Mexico, but repeated colonizations of cave environments have resulted in the independent evolution of troglomorphic phenotypes in several populations. Here, we present three chromosome-scale assemblies of this species, for one surface and two cave populations, enabling the first whole-genome comparisons between independently evolved cave populations to evaluate the genetic basis for the evolution of adaptation to the cave environment. Our assemblies represent the highest quality of sequence completeness with predicted protein-coding and non-coding gene metrics far surpassing prior resources and, to our knowledge, all long-read assembled teleost genomes, including zebrafish. Whole genome synteny alignments show highly conserved gene order among cave forms in contrast to a higher number of chromosomal rearrangements when compared to other phylogenetically close or distant teleost species. By phylogenetically assessing gene orthology across distant branches of amniotes, we discover gene orthogroups unique to A. mexicanus. When compared to a representative surface fish genome, we find a rich amount of structural sequence diversity, defined here as the number and size of insertions and deletions as well as expanding and contracting repeats across cave forms. These new more complete genomic resources ensure higher trait resolution for comparative, functional, developmental, and genetic studies of drastic trait differences within a species.
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Project description:BackgroundSeveral studies suggested that cavefish populations of Astyanax mexicanus settled during the Late Pleistocene. This implies that the cavefish's most conspicuous phenotypic changes, blindness and depigmentation, and more cryptic characters important for cave life, evolved rapidly.ResultsUsing the published genomes of 47 Astyanax cavefish from la Cueva de El Pachón, El Sótano de la Tinaja, La Cueva Chica and El Sótano de Molino, we searched for putative loss-of-function mutations in previously defined sets of genes, i.e., vision, circadian clock and pigmentation genes. Putative non-functional alleles for four vision genes were identified. Then, we searched genome-wide for putative non-functional alleles in these four cave populations. Among 512 genes with segregating putative non-functional alleles in cavefish that are absent in surface fish, we found an enrichment in visual perception genes. Among cavefish populations, different levels of shared putative non-functional alleles were found. Using a subset of 12 genes for which putative loss-of-function mutations were found, we extend the analysis of shared pseudogenes to 11 cave populations. Using a subset of six genes for which putative loss-of-function mutations were found in the El Sótano del Toro population, where extensive hybridization with surface fish occurs, we found a correlation between the level of eye regression and the amount of putative non-functional alleles.ConclusionsWe confirm that very few putative non-functional alleles are present in a large set of vision genes, in accordance with the recent origin of Astyanax mexicanus cavefish. Furthermore, the genome-wide analysis indicates an enrichment of putative loss-of-function alleles in genes with vision-related GO-terms, suggesting that visual perception may be the function chiefly impacted by gene losses related to the shift from a surface to a cave environment. The geographic distribution of putative loss-of-function alleles newly suggests that cave populations from Sierra de Guatemala and Sierra de El Abra share a common origin, albeit followed by independent evolution for a long period. It also supports that populations from the Micos area have an independent origin. In El Sótano del Toro, the troglomorphic phenotype is maintained despite massive introgression of the surface genome.
| S-EPMC10983663 | biostudies-literature
Project description:Organisms living in the subterranean biome evolve extreme characteristics including vision loss and sensory expansion. Despite prior work linking certain genes to Mendelian traits, the genetic basis for complex cave-associated traits remains unknown. Moreover, it is unclear if certain forms of genetic variation (e.g., indels, copy number variants) are more common in regressive evolution. Progress in this area has been limited by a lack of suitable natural model systems and genomic resources. In recent years, the Mexican tetra, Astyanax mexicanus, has advanced as a model for cave biology and regressive evolution. Here, we present the results of a genome-wide screen for in-frame indels using alignments of RNA-sequencing reads to the draft cavefish genome. Mutations were discovered in three genes associated with blood physiology (mlf1, plg, and wdr1), two genes associated with growth factor signaling (ghrb, rnf126), one gene linked to collagen defects (mia3), and one gene which may have a global epigenetic impact on gene expression (mki67). With one exception, polymorphisms were shared between Pachón and Tinaja cavefish lineages, and different from the surface-dwelling lineage. We confirmed the presence of mutations using direct Sanger sequencing and discovered remarkably similar developmental expression in both morphs despite substantial coding sequence alterations. Further, three mutated genes mapped near previously established quantitative trait loci associated with jaw size, condition factor, lens size, and neuromast variation. This work reveals previously unappreciated traits evolving in this species under environmental pressures (e.g., blood physiology) and provides insight to genetic changes underlying convergence of organisms evolving in complete darkness.
| S-EPMC6751357 | biostudies-literature