Polymorphism of the rDNA Chromosomal Regions in the Weatherfish Misgurnus fossilis (Teleostei: Cobitidae)

The weatherfish, Misgurnus fossilis (Linnaeus, 1758), is native and an endangered species in Europe. Individuals with a chromosome number of 100 representing the most frequent cytotype seem to have a polyploid origin in comparison with rarely observed individuals of 50 chromosomes. This study cytogenetically characterized M. fossilis (five males, seven females) possessing 100 chromosomes to show the chromosomal distribution of major and minor ribosomal DNAs, and telomeric DNA repeats using fluorescence in situ hybridization with 28S and 5S rDNAs, and telomere peptide nucleic acid probes, respectively. Silver nitrate (AgNO₃) staining was used to show the activity of nucleolus organizer regions (Ag-NORs) and chromomycin A₃ (CMA₃) fluorochrome staining to detect the GC-rich chromosome regions. In all individuals size polymorphism of Ag-NORs and rDNA clusters was identified. Most of the studied individuals exhibited four 28S rDNA sites found in the short arms of the submetacentric and subtelo-acrocentric chromosomes, however in one female only two 28S rDNA sites were observed. Chromosome regions consisting of 28S rDNA corresponded to AgNO₃ positive and GC-rich chromatin. The 5S rDNA loci were located predominantly in a pericentromeric position of eight or six subtelo-acrocentric chromosomes and did not co-localize with 28S rDNA. The telomeric repeats were exclusively localized at the ends of all M. fossilis chromosomes. The presence of four NOR-bearing chromosomes may reflect the polyploid origin of M. fossilis with 100 chromosomes. In turn, reduction of the NOR sites observed in one specimen might be considered as part of the rediploidization process.