Chromosomal rDNA Distribution Patterns in Clonal Cobitis Triploid Hybrids (Teleostei, Cobitidae): Insights into Parental Genomic Contributions

<b>Background:</b> Interspecific hybridization between relative species <i>Cobitis taenia</i> (with a diploid genome designated as TT), <i>Cobitis elongatoides</i> (EE) and <i>Cobitis tanaitica</i> (NN) and the successive polyploidization with transitions from sexuality to asexuality experienced by triploid <i>Cobitis</i> hybrids likely influence their chromosomal rearrangements, including rearrangements of ribosomal DNA (rDNA) distribution patterns. Previously, we documented distinct karyotypic differences: <i>C. elongatoides</i> exhibited bi-armed chromosomes while <i>C. taenia</i> showed uni-armed chromosomes with rDNA-positive hybridization signals, respectively. <b>Methods:</b> In this study, fluorescence in situ hybridization (FISH) with <i>5S</i> rDNA and <i>28S</i> rDNA probes was used to analyze and compare chromosomal distribution patterns of rDNAs in clonally reproduced triploid <i>Cobitis</i> hybrids of different genomic constitutions ETT, ETN, EEN and EET (referred to using acronyms denoting the haploid genomes of their parent species), and their parental species. <b>Results:</b><i>Cobitis</i> triploid hybrids exhibited intermediate karyotypes with ribosome synthesis sites on chromosomes inherited from both parents, showing no evidence of nucleolar dominance. The rDNA pattern derived from the <i>C. elongatoides</i> genome was more stable in the hybrids' karyotypes. Two and one submetacentric chromosomes with co-localized rDNAs were effective markers to ascertain <i>C. elongatoides</i> diploid (EE) and haploid (E) genomes within the genome of triploid hybrids, respectively. Fewer <i>5S</i> rDNA loci were observed in diploid (TT) and haploid (T) chromosome sets from <i>C. taenia</i> in ETT and EET females. <i>C. taenia</i> and <i>C. tanaitica</i> exhibited similar rDNA distribution patterns. <b>Conclusions:</b> The karyotypes of triploid <i>Cobitis</i> hybrids reflect the genomic contributions of their parental species. Variability in rDNA distribution patterns suggests complex genomic interactions in <i>Cobitis</i> hybrids resulting from polyploidization and hybridization, potentially influencing their reproductive potential.