The chromosomal distribution of ribosomal DNA and repetitive DNAs in the genome of the red cheek barb, Systomus rubripinnis (Valenciennes, 1842)

The distribution of 5S and 18S rDNA, as well as three microsatellites (di- and tri-nucleotide), on the chromosomes of Systomus rubripinnis (Valenciennes, 1842), commonly known as the red cheek barb, was analyzed using standard karyotyping techniques. Conventional staining techniques (Geimsa, C-banding, and Ag-NOR banding) and fluorescence in situ hybridization (FISH) were employed with 18S rDNA and microsatellite probes. The results showed that both males and females of the species carried identical karyotypes, with 2n = 50 chromosomes composed of 4 metacentric pairs, 5 submetacentric pairs, and 16 acrocentric pairs, including homomorphic sex chromosomes. As expected, C-positive heterochromatin was observed in the centromeric regions of all chromosomes. Additionally, NORs and satellite DNAs were mapped to the subtelomeric regions of acrocentric chromosomes, as well as to a conspicuous proximal heterochromatic block on the short arm of pairs No. 12 and 14. The distribution patterns of different ribosomal DNA (5S and 18S rDNAs) along the chromosomes varied considerably. The 18S rDNA sites were located on the telomeric regions of the short arms of chromosomes No. 12, 13, and 14, whereas the 5S rDNA showed a subtelomeric location on pair No. 10 and telomeric regions of pair No. 15. Strong hybridization signals were observed at the subtelomeric and telomeric regions of several acrocentric chromosomes. Fluorescence in situ hybridization (FISH) signals of the (CA)₁₅, (GA)₁₅, and (CGG)₁₀ sequences on the chromosomes were weak and dispersed, without preferential accumulations in any specific chromosomal pairs of the species.