Ribosomal genes in Coregonid fishes (Coregonus lavaretus, C. albula and C. peled) (Salmonidae): single and multiple nucleolus organizer regions.

Major rDNA loci, i.e. nucleolus-organizing regions (NORs), were assigned using chromomycin-A3 (CMA3) staining followed by sequential silver (Ag) staining and in situ hybridization (ISH) with a rDNA probe to the chromosomes of the European whitefish (Coregonus lavaretus), the peled (Coregonus peled) and the vendace (Coregonus albula), three closely related coregonine salmonid fishes. One pair of NOR-bearing chromosomes was found in the peled karyotype. Multichromosomal, but stable, locations of rDNA sites on three pairs of chromosomes were observed in the European whitefish karyotype. Multichromosomal polymorphic locations, both in site and number, were observed in the karyotype of the vendace. Several Ag-, CMA3- and ISH-positive regions were found which defined up to seven cytotypes of five NOR-bearing chromosomes. All positive Ag-NORs detected corresponded both to rDNA-ISH- and CMA3-positive signals, which suggests extensive structural polymorphism in the locations of rDNA sites. Stable NOR sites were found at the same location on both homologous elements of the chromosome no. 9 in all individuals, while the remaining NORs were quite variable between individuals, and often present in heterozygous condition. The apparently similar and parallel evolutionary rDNA differentiation patterns in the subfamilies Coregoninae and Salmoninae (family Salmonidae) are observed and discussed.