Phylogeography of an Amazonian Cichlid supports strong structuration by water current and past evolution by vicariance associated to the Amazon's formation.

Abstract

Amazonia is characterized by very heterogeneous riverscapes dominated by two drastically divergent water types: black (ion-poor, dissolved organic carbonate rich and acidic) and white (nutrient rich and turbid) waters. Recent phylogeographic and genomic studies have associated the ecotone formed by these environments to ecologically driven speciation in fish species. With the objective of better understanding the evolutionary forces behind the Amazonian Teleostean diversification, we sampled 240 Mesonauta festivus from 12 sites on a wide area of the Amazonian basin. These sites included three confluences of black and white water environments to seek for repeated evidences of ecological speciation at these ecotones. Our genetic dataset of 41,268 SNPs is contrasting with previous results and supports a low structuring power of water types. Conversely, we detected a strong pattern of isolation by unidirectional downstream water current and evidence of past events of vicariance potentially linked to the Amazon River formation and salt-water incursions that occurred 2.5 Mya. Using a combination of population genetic, phylogeographic analysis and environmental association models, we decomposed the spatial variance from the environmental genetic variance specifically to assess which evolutive forces have shaped inter-population differences in M. festivus’ genome. Our sampling design comprising four major Amazonian rivers and three confluences of black and white water rivers supports the possibility that past studies potentially confounded ecological speciation with a site effect unrepresentative of the full Amazonian watershed. While ecological speciation admittedly played a role in Amazonian fish species diversification, we argue that neutral evolutionary processes explain most of the divergence between M. festivus populations.

Gut microbiota of an Amazonian fish in a heterogeneous riverscape: integrating genotype, environment, and parasitic infections

“…In addition, parasitic infections by nematodes have previously been reported in wild specimens of M. festivus ( 44 ), and the species has important parental care investment, staying with fingerlings post hatching ( 43 ). Finally, in terms of genetic diversity, the four populations of M. festivus recorded in Amazonia are not structured according to the water physicochemical characteristics ( 45 ), therefore, making this species model suitable to disentangle environmental versus genetic effect on microbiota composition. Taken together, M. festivus is a relevant model to measure the relative contribution of environment, genotype, and parasitic infection in shaping the taxonomic structure of fish microbiota in a natural system.…”