Abstract
The Pampas is a Neotropical biome formed primarily by low altitude grasslands and encompasses the southernmost portion of Brazil, Uruguay, and part of Argentina. Despite the high level of endemism, and its significant environmental heterogeneity, Pampean species are underrepresented in phylogeographic studies, especially aquatic organisms. The Pampean hydrological system resulted from a long history of tectonism, climate, and sea level changes since the Neogene. In this study, we examined the population genetic structure of Cnesterodon decemmaculatus, a freshwater fish species that occurs throughout most of the Pampa biome. We characterized mitochondrial and autosomal genetic lineages in populations sampled from Southern Brazil and Uruguay to investigate (1) the correspondence between current drainage systems and evolutionary lineages, (2) the demographic history for each genetic lineage, and (3) the temporal depth of these lineages. Overall, we found that the major evolutionary lineages in this species are strongly related to the main Pampean drainage systems, even though stream capture events may have affected the distribution of genetic lineages among drainages. There was evidence for recent population growth in the lineages occupying drainages closest to the shore, which may indicate the effect of quaternary sea-level changes. In general, divergence time estimates among evolutionary lineages were shallow, ranging from 20,000 to 800,000 years before present, indicating a geologically recent history for this group, as previously reported in other Pampean species. A Bayesian phylogeographical reconstruction suggested that an ancestral lineage probably colonized the Uruguay River Basin, and then expanded throughout the Pampas. This evolutionary scenario may represent useful starting models for other freshwater species having a similar distribution.
Highlights
The South American Pampa, or “Pampas,” is a Neotropical biome dominated by natural grasslands spreading over plains of Uruguay, Northern Argentina, Southern Brazil, and part of Paraguay (Pallarés et al, 2005)
Because we found a general genetic structure associated to major Pampean drainages, we defined four geographical groups for population analysis: Uruguay River Basin (URU), Negro River Basin (NEG), Mirim Lagoon Basin (MIR), and Southern coastal Basins of Uruguay (SOU) (Figure 1 and Supplementary Table S1)
Ninety-nine sequences were obtained for the mtDNA NADH dehydrogenase 2 (ND2) gene (980 bp) yielding 31 different haplotypes defined by 42 variable sites (28 parsimoniously informative)
Summary
The South American Pampa, or “Pampas,” is a Neotropical biome dominated by natural grasslands spreading over plains of Uruguay, Northern Argentina, Southern Brazil, and part of Paraguay (Pallarés et al, 2005). While some phylogeographic studies in the Pampas have found little geographic structure and shallow gene trees [2.5 mya) (Felappi et al, 2015). These studies did not include freshwater organisms, and, it is difficult to predict the level and depth of the genetic structure exhibited by these species. Species occurring in the Pampa biome have been underrepresented in studies of phylogeography and conservation genetics (Lawler et al, 2006; Beheregaray, 2008; Turchetto-Zolet et al, 2013), which further complicates the understanding of general drivers of biological diversification in this biome
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