Genetic diversity of the swamp rat in South America: Population expansion after transgressive-regressive marine events in the Late Quaternary

Abstract We examined the phylogeography of the South American swamp rat Scapteromys tumidus using complete mitochondrial DNA cytochrome b sequences. This species is endemic to the Pampas biome and lives near the coastal plain but also in inland continental areas. The coastal domain of such region experienced a highly dynamic geological history. The inland fraction is part of Precambrian and Mesozoic spills and sedimentary basins, while most of the coastal plain was shaped by transgressive-regressive marine events in the Late Quaternary. As fluctuations in the sea level during this period produced large lateral displacements of the shoreline, originating four barrier–lagoon systems that shaped the present coastline, continental area significantly increased from this time. We hypothesized that the S. tumidus have expanded its range from that period, ramped by its adaptive ability to humid areas. Thus, we examined whether historical Pleistocene events had affected genetic variation of this species along its distributional range. Bayesian phylogenetic analysis and the haplotype network inferred two major genetic clusters along the distribution of S. tumidus . Neutrality tests suggest that populations experienced demographic changes and Bayesian skyline plot evidenced a marked recent demographic expansion pattern, intensified from 350 to 300 kya. Such date coincides with the formation of the lagunar barrier II (dated to ca. 325 kya) and the concurrent initial formation of the coastal plain and increase in continental area.