Maintenance of an endemic lineage of brown trout (Salmo trutta) within the Duero river basin

The Iberian Peninsula contains diverse populations of freshwater fish, with major river basins comprising differentiated biogeographic units. The Duero River flows through the North-Western Iberian Peninsula and is one of the most important rivers within the Iberian glacial refuge. Brown trout (Salmo trutta) populate this whole basin, and studies using both allozyme and microsatellite loci have detected a geographically sorted distribution of genetic variation in this species. In this work, sequences of the mitochondrial control region obtained from 299 brown trout from the Duero River were compared with other Iberian and European datasets. Two differentiated haplotype groups were detected inside the Duero River basin. One of them was related to the Atlantic (AT) lineage that is present in Northern European populations, whereas the other comprised an unique group that was restricted to the inner region of the basin. The amount of divergence of this Duero group from the other brown trout populations studied is consistent with a new trout lineage (Duero, DU) that is endemic to this river basin and that diverged from other Atlantic populations during the Pleistocene. The distribution of the DU and AT quaternary lineages in the Duero River was consistent with the ichthyological pattern described in the basin that originated during the Miocene-Pliocene. Evidence of selective processes that favour the haplotypes of the DU lineage may explain this discrepancy.