Past hybridisation and introgression erased traces of mitochondrial lineages evolution in the Neotropical silver catfish Rhamdia quelen (Siluriformes: Heptapteridae)

The Neotropical species complex Rhamdia quelen is composed of at least six mitochondrial lineages. Three of these occur in sympatry in several regional basins, which encompass La Plata basin, Patos-Merin basin system and the coastal lagoons draining to SW Atlantic Ocean. Based on both mitochondrial cytochrome b gene and 10 nuclear microsatellite loci markers, this study aims to investigate the genetic diversity pattern and the reproductive isolation among R. quelen mitochondrial lineages. Past hybridisation and introgression were evidenced among at least two mitochondrial lineages. The ancestral structure recovered in this study was divided into two groups, which could have diverged in the last marine regression. The recent population structure of R. quelen species complex is mostly recovered following the geographic distribution pattern. We delimited seven management units, three inhabiting riverine environments and four associated to different coastal lagoons. Lagoon populations, unlike the riverine ones, would have diverged in a scenario with null or restricted gene flow and small size population, possibly related to bottleneck events. Population genetic structure should be considered for conservation legislation, fishery management and aquaculture regulation. Additionally, the present genetic structure could aggravate the impact of specimen translocation and escapees from aquaculture farms.