Hybridization is strongly constrained by salinity during secondary contact between silverside fishes (Odontesthes, Atheriniformes).

This study investigates a contact zone between two silverside fish species (marine Odontesthes argentinensis and freshwater O. bonariensis) in the estuarine Mar Chiquita lagoon along the Atlantic coast in Argentina (MChL), in which intermediate morphs had been reported. It has been suggested that admixture and introgression occur in MChL between these two species, but direct genetic evidence is lacking. Leveraging samples collected over several years (n = 676), we document the spatial distribution of both species and intermediate morphs within this habitat and collect landmark-based morphometric and multilocus genetic data (9876 loci for n = 110 individuals) to test the hypothesis of hybridization. Our analysis unambiguously characterizes intermediate morphs as F1 or F2 hybrids. We show that the low frequency of hybrid individuals in MChL may be explained by uneven abundance of parental species, which in turn are strongly affected by water salinity, limiting the size of the contact zone. Although hybrids seem to be fertile, their fitness may be reduced by external and intrinsic factors that may limit their success and suggest that this is an unstable hybrid zone. Genetic distinctiveness of both parental species is strongly supported by genome-wide data, explaining a known pattern of mitonuclear discordance as a consequence of hybridization followed by mitochondrial introgression. A clear signature of population genetic structure was detected in O. argentinensis, distinguishing MChL residents from marine populations of this species, that also was supported by distinctive morphometric features among these groups. Previous hypotheses of speciation in these fishes are discussed in the light of the new findings.

Biogeography, habitat transitions and hybridization in a radiation of South American silverside fishes revealed by mitochondrial and genomic RAD data.

Rivers and lake systems in the southern cone of South America have been widely influenced by historical glaciations, carrying important implications for the evolution of aquatic organisms, including prompting transitions between marine and freshwater habitats and by triggering hybridization among incipient species via waterway connectivity and stream capture events. Silverside fishes (Odontesthes) in the region comprise a radiation of 19 marine and freshwater species that have been hypothesized on the basis of morphological or mitochondrial DNA data to have either transitioned repeatedly into continental waters from the sea or colonized marine habitats following freshwater diversification. New double digest restriction-site associated DNA data presented here provide a robust framework to investigate the biogeographical history of and habitat transitions in Odontesthes. We show that Odontesthes silversides originally diversified in the Pacific but independently colonized the Atlantic three times, producing three independent marine-to-freshwater transitions. Our results also indicate recent introgression of marine mitochondrial haplotypes into two freshwater clades, with more recurring instances of hybridization among Atlantic- versus Pacific-slope species. In Pacific freshwater drainages, hybridization with a marine species appears to be geographically isolated and may be related to glaciation events. Substantial structural differences of estuarine gradients between these two geographical areas may have influenced the frequency, intensity and evolutionary effects of hybridization events.

An integrated approach clarifies the cryptic diversity in Hypostomus Lacépède 1803 from the Lower La Plata Basin.

Hypostomus commersoni Valenciennes 1836, Hypostomus cordovae (Günther 1880) and Hypostomus laplatae (Eigenmann 1907) have been little studied since their original descriptions. This study shows a comprehensive review of these species from the Lower La Plata Basin, including their taxonomic history, distribution, color patterns, morphology, and ecological and molecular phylogenetic data. Morphological and phylogenetic analyses based on D-loop sequences suggested that H. commersoni can be separated into two subclades, or subgroups. Based on these results and on the non-overlapping distribution range of the two subclades, we conclude that they represent two distinct species, thereby revalidating H. spiniger. The results also suggest that H. paranensis should be considered as species inquirenda and H. cordovae as valid species. This integrated approach provides key information for assessing the conservation status and biogeographic aspects of the genus Hypostomus in the Lower La Plata Basin.

Molecular and morphological evidence revalidates Acrobrycontarijae (Characiformes, Characidae) and shows hidden diversity.

We conducted a revision of the Neotropical genus Acrobrycon. A previous study synonymized the species, A.ipanquianus, distributed from the western portion of the Amazon River to the north-western region of the La Plata River Basin, and A.tarijae, with type locality in the Lipeo River in Bolivia. We revisited this result by collecting new morphometric, meristic, and genetic data (COI mitochondrial gene) for 24 individuals distributed along La Plata River Basin in Argentina, and discussed our results in the context of multiple biogeographic processes of isolation in that basin. Our results revealed a more complex history of diversification and geographic distribution across Acrobrycon species than previously suspected, probably associated with multiple biogeographic processes of isolation in La Plata River Basin. We present new evidence that led us to reconsider the validity of A.tarijae, which is distinguishable from A.ipanquianus by the number of vertebrae (37-39 vs. 41-42) and pleural ribs (12-13 vs. 14). These results were also supported by our molecular analyses that revealed a genetic divergence >4% between A.ipanquianus and A.tarijae. We also identified two main genetic clusters within A.tarijae: the first cluster consisted of specimens from the Bermejo, Pilcomayo, Itiyuro and Juramento river basins (northern Argentina); and the second cluster included specimens from the southernmost basins, such as the Salí River in Tucumán, Cuarto River in the province of Cordoba and the Quinto River in the province of San Luis. Our results suggest that the genetic structure observed in A.tarijae is the result of the type of drainage (endorheic vs. exorheic) and geographical distance.

Multilocus phylogeny and historical biogeography of Hypostomus shed light on the processes of fish diversification in La Plata Basin.

Distribution history of the widespread Neotropical genus Hypostomus was studied to shed light on the processes that shaped species diversity. We inferred a calibrated phylogeny, ancestral habitat preference, ancestral areas distribution, and the history of dispersal and vicariance events of this genus. The phylogenetic and distribution analyses indicate that Hypostomus species inhabiting La Plata Basin do not form a monophyletic clade, suggesting that several unrelated ancestral species colonized this basin in the Miocene. Dispersal to other rivers of La Plata Basin started about 8 Mya, followed by habitat shifts and an increased rate of cladogenesis. Amazonian Hypostomus species colonized La Plata Basin several times in the Middle Miocene, probably via the Upper Paraná and the Paraguay rivers that acted as dispersal corridors. During the Miocene, La Plata Basin experienced marine incursions, and geomorphological and climatic changes that reconfigured its drainage pattern, driving dispersal and diversification of Hypostomus. The Miocene marine incursion was a strong barrier and its retraction triggered Hypostomus dispersal, increased speciation rate and ecological diversification. The timing of hydrogeological changes in La Plata Basin coincides well with Hypostomus cladogenetic events, indicating that the history of this basin has acted on the diversification of its biota.

Processes that drive the population structuring of Jenynsia lineata (Cyprinidontiformes, Anablepidae) in the La Plata Basin.

The distribution of genetic diversity across a species distribution range is rarely homogeneous, as the genetic structure among populations is related to the degree of isolation among them, such as isolation by distance, isolation by barrier, and isolation by environment. Jenynsia lineata is a small viviparous fish that inhabits a wide range of habitats in South America. To decipher the isolation processes that drive population structuring in J. lineata, we analyzed 221 sequences of the mitochondrial cytochrome c oxidase I gene (COI), from 19 localities. Then, we examined the influence of the three most common types of isolation in order to explain the genetic variation found in this species.Our results revealed a marked structuration, with three groups: (a) La Plata/Desaguadero Rivers (sampling sites across Argentina, Uruguay, and Southern Brazil), (b) Central Argentina, and (c) Northern Argentina. A distance-based redundancy analysis, including the explanatory variables geographical distances, altitude, latitude, and basin, was able to explain up to 65% of the genetic structure. A variance partitioning analysis showed that the two most important variables underlying the structuration in J. lineata were altitude (isolation by environment) and type of basin (isolation by barrier).Our results show that in this species, the processes of population diversification are complex and are not limited to a single mechanism. The processes that play a prominent role in this study could explain the high rate of diversity that characterizes freshwater fish species. And these processes in turn are the basis for possible speciation events.

Unexpected diversity in the catfish Pseudancistrus brevispinis reveals dispersal routes in a Neotropical center of endemism: the Guyanas Region.

Neotropical freshwater fishes have reached an unrivalled diversity, organized into several areas of endemism, yet the underlying processes are still largely unknown. The topographical and ecological characteristics of the Guyanas Region make it an ideal area of endemism in which to investigate the forces that have shaped this great diversity. This region is thought to be inhabited by species descending from Amazonian ancestors, which would have used two documented routes that, however, hardly explain the entrance of species adapted to running waters. Here, we investigate the evolutionary history of Pseudancistrus brevispinis, a catfish endemic to this region and exclusively found in running waters, thus making it an ideal model for investigating colonization routes and dispersal in such habitats. Our analyses, based on mitochondrial and nuclear markers, revealed an unexpected diversity consisting of six monophyletic lineages within P. brevispinis, showing a disjoint distribution pattern. The lineages endemic to Guyanas coastal rivers form a monophyletic group that originated via an ancestral colonization event from the Amazon basin. Evidence given favours a colonization pathway through river capture between an Amazonian tributary and the Upper Maroni River. Population genetic analyses of the most widespread species indicate that subsequent dispersal among Guyanas coastal rivers occurred principally by temporary connections between adjacent rivers during periods of lower sea level, yet instances of dispersal via interbasin river captures are not excluded. During high sea level intervals, the isolated populations would have diverged leading to the observed allopatric species. This evolutionary process is named the sea level fluctuation (SLF) hypothesis of diversification.

A continental-wide molecular approach unraveling mtDNA diversity and geographic distribution of the Neotropical genus Hoplias.

With an estimate of around 9,000 species, the Neotropical region hosts the greatest diversity of freshwater fishes of the world. Genetic surveys have the potential to unravel isolated and unique lineages and may result in the identification of undescribed species, accelerating the cataloguing of extant biodiversity. In this paper, molecular diversity within the valuable and widespread Neotropical genus Hoplias was assessed by means of DNA Barcoding. The geographic coverage spanned 40 degrees of latitude from French Guiana to Argentina. Our analyses revealed 22 mitochondrial lineages fully supported by means of Barcode Index Number, Automatic Barcode Gap Discovery and phylogenetic analyses. This mtDNA survey revealed the existence of 15 fully supported mitochondrial lineages within the once considered to be the continentally distributed H. malabaricus. Only four of them are currently described as valid species however, leaving 11 mitochondrial lineages currently "masked" within this species complex. Mean genetic divergence was 13.1%. Barcoding gap analysis discriminated 20 out of the 22 lineages tested. Phylogenetic analyses showed that all taxonomically recognized species form monophyletic groups. Hoplias malabaricus sensu stricto clustered within a large clade, excluding the representatives of the La Plata River Basin. In the H. lacerdae group, all species but H. curupira showed a cohesive match between taxonomic and molecular identification. Two different genetic lineages were recovered for H. aimara. Given the unexpected hidden mitochondrial diversity within H. malabaricus, the COI sequence composition of specimens from Suriname (the type locality), identified as H. malabaricus sensu stricto, is of major importance.