Testing Wallace's intuition: water type, reproductive isolation and divergence in an Amazonian fish.

Alfred Russel Wallace proposed classifying Amazon rivers based on their colour and clarity: white, black and clear water. Wallace also proposed that black waters could mediate diversification and yield distinct fish species. Here, we bring evidence of speciation mediated by water type in the sailfin tetra (Crenuchus spilurus), a fish whose range encompasses rivers of very distinct hydrochemical conditions. Distribution of the two main lineages concords with Wallace's water types: one restricted to the acidic and nutrient-poor waters of the Negro River (herein Rio Negro lineage) and a second widespread throughout the remaining of the species' distribution (herein Amazonas lineage). These lineages occur over a very broad geographical range, suggesting that despite occurring in regions separated by thousands of kilometres, individuals of the distinct lineages fail to occupy each other's habitats, hundreds of metres apart and not separated by physical barrier. Reproductive isolation was assessed in isolated pairs exposed to black-water conditions. All pairs with at least one individual of the lineage not native to black waters showed significantly lower spawning success, suggesting that the water type affected the fitness and contributed to reproductive isolation. Our results endorse Wallace's intuition and highlight the importance of ecological factors in shaping diversity of the Amazon fish fauna.

Preliminary profile of the gut microbiota from amerindians in the Brazilian amazon experiencing a process of transition to urbanization.

The Yanomami are one of the oldest indigenous tribes in the Amazon and are direct descendants of the first people to colonize South America 12,000 years ago. They are located on the border between Venezuela and Brazil, with the Venezuelan side remaining uncontacted. While they maintain a hunter-gatherer society, they are currently experiencing contact with urbanized populations in Brazil. The human gut microbiota of traditional communities has become the subject of recent studies due to the Westernization of their diet and the introduction of antibiotics and other chemicals, which have affected microbial diversity in indigenous populations, thereby threatening their existence. In this study, we preliminarily characterized the diversity of the gut microbiota of the Yanomami, a hunter-gatherer society from the Amazon, experiencing contact with urbanized populations. Similarly, we compared their diversity with the population in Manaus, Amazonas. A metabarcoding approach of the 16 S rRNA gene was carried out on fecal samples. Differences were found between the two populations, particularly regarding the abundance of genera (e.g., Prevotella and Bacteroides) and the higher values of the phyla Bacteroidetes over Firmicutes, which were significant only in the Yanomami. Some bacteria were found exclusively in the Yanomami (Treponema and Succinivibrio). However, diversity was statistically equal between them. In conclusion, the composition of the Yanomami gut microbiota still maintains the profile characteristic of a community with a traditional lifestyle. However, our results suggest an underlying Westernization process of the Yanomami microbiota when compared with that of Manaus, which must be carefully monitored by authorities, as the loss of diversity can be a sign of growing danger to the health of the Yanomami.

One thousand DNA barcodes of piranhas and pacus reveal geographic structure and unrecognised diversity in the Amazon.

Piranhas and pacus (Characiformes: Serrasalmidae) are a charismatic but understudied family of Neotropical fishes. Here, we analyse a DNA barcode dataset comprising 1,122 specimens, 69 species, 16 genera, 208 localities, and 34 major river drainages in order to make an inventory of diversity and to highlight taxa and biogeographic areas worthy of further sampling effort and conservation protection. Using four methods of species discovery-incorporating both tree and distance based techniques-we report between 76 and 99 species-like clusters, i.e. between 20% and 33% of a priori identified taxonomic species were represented by more than one mtDNA lineage. There was a high degree of congruence between clusters, with 60% supported by three or four methods. Pacus of the genus Myloplus exhibited the most intraspecific variation, with six of the 13 species sampled found to have multiple lineages. Conversely, piranhas of the Serrasalmus rhombeus group proved difficult to delimit with these methods due to genetic similarity and polyphyly. Overall, our results recognise substantially underestimated diversity in the serrasalmids, and emphasise the Guiana and Brazilian Shield rivers as biogeographically important areas with multiple cases of across-shield and within-shield diversifications. We additionally highlight the distinctiveness and complex phylogeographic history of rheophilic taxa in particular, and suggest multiple colonisations of these habitats by different serrasalmid lineages.

A new species of Tometes Valenciennes 1850 (Characiformes: Serrasalmidae) from Tocantins-Araguaia River Basin based on integrative analysis of molecular and morphological data.

A new large serrasalmid species of Tometes is described from the Tocantins-Araguaia River Basin. Tometes siderocarajensis sp. nov. is currently found in the rapids of the Itacaiúnas River Basin, and formerly inhabited the lower Tocantins River. The new species can be distinguished from all congeners, except from T. ancylorhynchus, by the presence of lateral space between 1st and 2nd premaxillary teeth, and by the absence of lateral cusps in these two teeth. However, T. siderocarajensis sp. nov. can be differentiated from syntopic congener T. ancylorhynchus by an entirely black with mottled red body in live specimens, densely pigmented pelvic fins with a high concentration of dark chromatophores, and the presence of 39 to 41 rows of circumpeduncular scales (vs. silvery body coloration with slightly reddish overtones on middle flank during breeding period in live specimens, hyaline to slightly pale coloration on distalmost region of pelvic fins, and 30 to 36 rows of circumpeduncular scales). Additionally, molecular sequence shows that T. siderocarajensis sp. nov. is reciprocally monophyletic, and diagnosable from all congeners by having two autapomorphic molecular characters in the mitochondrial gene COI. The phylogenetic reconstruction still show that T. siderocarajensis sp. nov. is closely related to T. trilobatus. This is the first molecular study using an integrative taxonomic approach based on morphological and molecular sequence data for all described species of Tometes. These findings increase the number of formally described species of Tometes to seven. A key to the Tometes species is provided.