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.

We investigated replicated ecological speciation in the livebearing fish Poecilia mexicana and P. sulphuraria (Poeciliidae), which inhabit freshwater habitats and have also colonized multiple sulfidic springs in southern Mexico. These springs exhibit extreme hypoxia and high concentrations of hydrogen sulfide, which is lethal to most metazoans. We used phylogenetic analyses to test whether springs were independently colonized, performed phenotypic assessments of body and gill morphology variation to identify convergent patterns of trait differentiation, and conducted an eco-toxicological experiment to detect differences in sulfide tolerances among ecotypes. Our results indicate that sulfidic springs were colonized by three different lineages, two within P. mexicana and one representing P. sulphuraria. Colonization occurred earlier in P. sulphuraria, whereas invasion of sulfidic springs in P. mexicana was more recent, such that each population is more closely related to neighboring populations from adjacent nonsulfidic habitats. Sulfide spring fish also show divergence from nonsulfidic phenotypes and a phenotypic convergence toward larger heads, larger gills, and increased tolerance to H 2 S. Together with previous studies that indicated significant reproductive isolation between fish from sulfidic and nonsulfidic habitats, this study provides evidence for repeated ecological speciation in the independent sulfide spring populations of P. mexicana and P. sulphuraria. K E Y W O R D S :Ecological speciation, hydrogen sulfide, hypoxia, local adaptation, morphological differentiation, mtDNA phylogeny, natural selection, Poecilia.

Divergent natural selection drives evolutionary diversification. It creates phenotypic diversity by favoring developmental plasticity within populations or genetic differentiation and local adaptation among populations. We investigated phenotypic and genetic divergence in the livebearing fish Poecilia mexicana along two abiotic environmental gradients. These fish typically inhabit nonsulfidic surface rivers, but also colonized sulfidic and cave habitats. We assessed phenotypic variation among a factorial combination of habitat types using geometric and traditional morphometrics, and genetic divergence using quantitative and molecular genetic analyses. Fish in caves (sulfidic or not) exhibited reduced eyes and slender bodies. Fish from sulfidic habitats (surface or cave) exhibited larger heads and longer gill filaments. Common-garden rearing suggested that these morphological differences are partly heritable. Population genetic analyses using microsatellites as well as cytochrome b gene sequences indicate high population differentiation over small spatial scale and very low rates of gene flow, especially among different habitat types. This suggests that divergent environmental conditions constitute barriers to gene flow. Strong molecular divergence over short distances as well as phenotypic and quantitative genetic divergence across habitats in directions classic to fish ecomorphology suggest that divergent selection is structuring phenotypic variation in this system.

Most eucaryotic organisms classified as living in an extreme habitat are invertebrates. Here we report of a fish living in a Mexican cave (Cueva del Azufre) that is rich in highly toxic H(2)S. We compared the water chemistry and fish communities of the cave and several nearby surface streams. Our study revealed high concentrations of H(2)S in the cave and its outflow (El Azufre). The concentrations of H(2)S reach more than 300 muM inside the cave, which are acutely toxic for most fishes. In both sulfidic habitats, the diversity of fishes was heavily reduced, and Poecilia mexicana was the dominant species indicating that the presence of H(2)S has an all-or-none effect, permitting only few species to survive in sulfidic habitats. Compared to habitats without H(2)S, P. mexicana from the cave and the outflow have a significantly lower body condition. Although there are microhabitats with varying concentrations of H(2)S within the cave, we could not find a higher fish density in areas with lower concentrations of H(2)S. We discuss that P. mexicana is one of the few extremophile vertebrates. Our study supports the idea that extreme habitats lead to an impoverished species diversity.

The family Mugilidae comprises mainly coastal marine species that are widely distributed in all tropical, subtropical and temperate seas. Mugilid species are generally considered to be ecologically important and they are a major food resource for human populations in certain parts of the world. The taxonomy and systematics of the Mugilidae are still much debated and based primarily on morphological characters. In this study, we provide the first comprehensive molecular systematic account of the Mugilidae using phylogenetic analyses of nucleotide sequence variation at three mitochondrial loci (16S rRNA, cytochrome oxidase I, and cytochrome b) for 257 individuals from 55 currently recognized species. The study covers all 20 mugilid genera currently recognized as being valid. The family comprises seven major lineages that radiated early on from the ancestor to all current forms. All genera that were represented by two species or more, except Cestraeus, turned out to be paraphyletic or polyphyletic. Thus, the present phylogenetic results generally disagree with the current taxonomy at the genus level and imply that the anatomical characters used for the systematics of the Mugilidae may be poorly informative phylogenetically. The present results should provide a sound basis for a taxonomic revision of the mugilid genera. A proportion of the species with large distribution ranges (including Moolgarda seheli, Mugil cephalus and M. curema) appear to consist of cryptic species, thus warranting further taxonomic and genetic work at the infra-generic level.