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Background: Interspecific hybridization between relative species Cobitis taenia (with a diploid genome designated as TT), Cobitis elongatoides (EE) and Cobitis tanaitica (NN) and the successive polyploidization with transitions from sexuality to asexuality experienced by triploid Cobitis hybrids likely influence their chromosomal rearrangements, including rearrangements of ribosomal DNA (rDNA) distribution patterns. Previously, we documented distinct karyotypic differences: C. elongatoides exhibited bi-armed chromosomes while C. taenia showed uni-armed chromosomes with rDNA-positive hybridization signals, respectively. Methods: In this study, fluorescence in situ hybridization (FISH) with 5S rDNA and 28S rDNA probes was used to analyze and compare chromosomal distribution patterns of rDNAs in clonally reproduced triploid Cobitis 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. Results: Cobitis 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 C. elongatoides genome was more stable in the hybrids’ karyotypes. Two and one submetacentric chromosomes with co-localized rDNAs were effective markers to ascertain C. elongatoides diploid (EE) and haploid (E) genomes within the genome of triploid hybrids, respectively. Fewer 5S rDNA loci were observed in diploid (TT) and haploid (T) chromosome sets from C. taenia in ETT and EET females. C. taenia and C. tanaitica exhibited similar rDNA distribution patterns. Conclusions: The karyotypes of triploid Cobitis hybrids reflect the genomic contributions of their parental species. Variability in rDNA distribution patterns suggests complex genomic interactions in Cobitis hybrids resulting from polyploidization and hybridization, potentially influencing their reproductive potential. Full article

Teleost fishes exhibit extraordinary diversity, plasticity and adaptability with their sex determination and sexual development, and there is growing evidence that non-coding RNAs (ncRNAs) are emerging as critical regulators of reproduction. Japanese flounder (Paralichthys olivaceus) is an important marine cultured fish that presents significant sexual dimorphism with bigger females, in which gynogenesis has been applied for aquaculture industry. In order to reveal the regulatory mechanisms of sexual development in gynogenetic female and sex-reversed neo-male P. olivaceus, the lncRNA–miRNA–mRNA interactions were investigated using high-throughput sequencing. A total of 6772 differentially expressed mRNAs (DEmRNAs), 2284 DElncRNAs, and 244 DEmiRNAs were obtained between gynogenetic female ovaries and sex-reversed neo-male testes. Genes in the steroid hormone biosynthesis and secretion pathway were enriched and mostly significantly upregulated in neo-male testes. Subsequently, network analysis uncovered high functional specificity for gynogenetic P. olivaceus sperm motility, as co-expressed DEmRNAs were significantly enriched in microtubule and cytoskeleton-related biological processes. Clustered miRNAs were characterized in the P. olivaceus genome with examples of the largest conserved let-7 clusters. The 20 let-7 members are distributed in 11 clusters and may not transcribe together with their neighboring miR-125b, with let-7 repressing cyp11a and miR-125b repressing esr2b, both as key steroidogenesis pathway genes. In summary, this study provides comprehensive insights into the mRNA–miRNA–lncRNA functional crosstalk in teleost sexual development and gametogenesis and will expand our understanding of ncRNA biology in teleost gynogenesis. Full article