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.