First cytogenetic characterization of the southern pygmy perch Nannoperca australis (Teleostei: Percichthyidae): Karyotype, heterochromatin and repetitive DNA organization.

Chromosome organization and repetitive DNA distribution are important components of genome evolution in teleost fishes but remain poorly characterized in many species of ecological and conservation relevance. We performed a detailed cytogenetic analysis of the southern pygmy perch, Nannoperca australis, a small freshwater species widely used in studies of population structure and evolutionary ecology in southeastern Australia. Mitotic chromosomes were examined using Giemsa staining, C-banding, CMA3 fluorochrome staining, fluorescence in situ hybridization (FISH) with 5S and 28S rDNA probes, telomeric FISH and comparative genomic hybridization (CGH). While the diploid number was conserved (2n = 48), reflecting a broader stability within the Percichthyidae, we observed minor morphological variation likely due to differential chromatin condensation. No heteromorphic sex chromosomes were detected via CGH, suggesting a homomorphic state common in many teleost lineages. However, 28S rDNA loci showed pronounced inter-individual variability, including a unique interstitial cluster in one male. This coexistence of a stable 2n = 48 template with highly dynamic rDNA clusters suggests that repetitive DNA turnover may drive individual-level genomic diversity in isolated populations. These findings provide a preliminary cytogenetic baseline for N. australis and highlight the potential of repetitive DNA as a marker for population structuring and conservation management in fragmented Australian catchments.