Heterochromatin Bands and rDNA Sites Evolution in Polyploidization Events in Cynodon Rich. (Poaceae)

Cynodon is a genus with a wide distribution in tropical and subtropical areas. Ploidy levels in Cynodon range from diploid to hexaploid; hence, polyploidy is the most important driver of chromosome number variation in this genus. After polyploidization, structural rearrangements such as deletions, insertion, or duplications of DNA sequences frequently occur, allowing variation of chromosome morphology, size, and number. These events might result in a wide diversity of karyotypes, contributing to reproductive isolation, and consequently to speciation. In this study, we investigate the karyotype variation of Cynodon based on comparative cytogenetic analyses. We conducted chromosome counts, DNA quantification, CMA/DAPI double staining, and FISH mapping of 5S and 35S rDNA sites. Cytomolecular data were analyzed in a phylogenetic framework, in order to trace the evolutionary history of the karyotype variation considering polyploidy events in this group. Our results indicate that the most recent common ancestor of Cynodon had two 35S and 5S rDNA sites, two CMA bands, nine DAPI+ bands on the long arm, five DAPI+ bands on the short arm, and three DAPI+ bands in the pericentromeric region. During polyploidization events, there were losses and gains of heterochromatic sequences mainly on the short arms and centromeric regions.