High Ag-NOR-site variation associated to a secondary contact in brown trout from the Iberian Peninsula

The analysis of nucleolar organizer regions (NORs) using silver (Ag-) staining and in situ hybridization (ISH) in brown trout (Salmo trutta) from various river basins in the Iberian Peninsula revealed high variation in the number and location of NORs. A total of 17 different Ag-NOR sites were revealed in 10 different chromosome pairs. Three different Ag-NOR patterns clustered by river basins and strongly associated to the internal transcribed spacer 1 (ITS1) variation were detected. The main variability in NOR-sites was found in a secondary contact between two divergent lineages of brown trouts at Duero basin. Our results confirmed the abrupt break in the spatial distribution of genetic variation of brown trout populations previously reported at Duero basin. We hypothesize that NOR-site variation might be a consequence of hybridization between divergent lineages of brown trouts and that NORs could play a major role in the maintenance of a hybrid zone in Duero basin via post-zygotic isolation mechanisms.