Karyotype and Repetitive DNA Analysis in Turcichondrostoma fahirae (Cypriniformes, Leuciscidae): A Step toward the Use of Molecular Cytogenetics in Taxonomy of Freshwater Fishes in Türkiye

Introduction: Türkiye houses rich freshwater ichthyofauna with many endemic species. This diversity, however, poses taxonomic challenges and leads to ongoing reevaluations of various fish genera and species. Here, we sought to analyze the karyotype and other chromosomal characteristics of the newly erected monotypic genus Turcichondrostoma to produce and validate cytogenetic markers potentially informative for future comparative studies. Methods: We examined an endemic species Turcichondrostoma fahirae (Tefenni nase) using conventional karyotyping and chromosome banding procedures (C-, fluorescent, and silver-nitrate banding/staining), as well as chromosomal mapping of 5S/18S ribosomal DNA (rDNA), U1/U2 small nuclear DNA (snDNA), and telomeric repeats. Results: A diploid chromosome number (2n) of T. fahirae was 50, consistent with conservative leuciscid pattern. The karyotype was composed of 12 metacentric, 22 submetacentric, 10 subtelocentric, and 6 acrocentric chromosomes. Low amount of constitutive heterochromatin was distributed almost exclusively across the pericentromeric regions of all chromosomes, with themost prominent C-bands being placed on a single chromosome pair carrying nucleolar organizer region (NOR). NORs (visualized consistently by silvernitrate staining, chromomycin A3, and fluorescence in situ hybridization) exhibited marked size heteromorphism and were adjacent to a more centromere-proximal 5S rDNA site on the long arm. Additional 5S rDNA clusters occupied short arms of four acrocentric chromosomes, and another single subtelocentric pair carried a single colocalized U1/U2 snDNA site. No interstitial telomeric sequences were detected. Conclusion: We performed a pioneer molecular cytogenetic study in Turkish freshwater fish species and our data suggest that molecular cytogenetic markers will aid in future taxonomic comparisons. Our findings further corroborate conserved karyotype structure of leuciscid fishes in general.