Karyotype analysis and chromosome evolution in Menyanthaceae using FISH

The Menyanthaceae, an aquatic plant family, is distinguished by extensive polyploidy and heterostyly. This study marks the first cytogenetic characterization of four Menyanthaceae species from Korea – Menyanthes trifoliata, Nymphoides peltata, N. indica, and N. coreana – employing fluorescence in situ hybridization (FISH) with 45S and 5S rDNA probes. All four species exhibit exclusively metacentric chromosomes, with M. trifoliata and N. peltata being hexaploid (2n = 54), N. coreana tetraploid (2n = 36), and N. indica diploid (2n = 18). FISH mapping revealed between one to four 45S rDNA loci and one to three 5S rDNA loci per species, showing that rDNA site number does not correlate directly with ploidy level. The karyotypic data suggest a conserved base chromosome number (x = 9) and largely symmetrical karyotypes across these species. Notably, M. trifoliata presents fewer rDNA loci than expected for a hexaploid, indicating genomic rearrangements and rDNA locus loss through diploidization. These observations highlight an evolutionarily stable genome structure in M. trifoliata, despite its polyploid nature. This study elucidates the chromosomal organization and evolutionary dynamics of the Menyanthaceae, emphasizing the role of polyploidy and rDNA evolution in genome structuring. The findings enhance our understanding of plant cytogenetics in aquatic ecosystems and serve as a foundation for further comparative genomic and evolutionary studies in Menyanthaceae.