Centromeric and non-centromeric satellite DNA organisation in holocentric Rhynchospora species

Satellite DNA repeats (or satDNA) are fastevolving sequences usually associated with condensed heterochromatin. To test whether the chromosomal organisation ofcentromeric and non-centromeric satDNA differs in specieswith holocentric chromosomes, we identified andcharacterised the major satDNA families in the holocentricCyperaceae species Rhynchospora ciliata (2n = 10), R.globosa (2n = 50) and R. tenuis (2n = 2x = 4 and2n = 4x = 8). While conserved centromeric repeats (presentin R. ciliata and R. tenuis) revealed linear signals at bothchromatids, non-centromeric, species-specific satDNAsformed distinct clusters along the chromosomes.Colocalisation of both repeat types resulted in a ladder-likehybridisation pattern at mitotic chromosomes. In interphase,the centromeric satDNAwas dispersed while non-centromericsatDNA clustered and partly colocalised to chromocentres.Despite the banding-like hybridisation patterns of the clustered satDNA, the identification of chromosome pairs wasimpaired due to the irregular hybridisation patterns of the homologues in R. tenuis and R. ciliata. These differences areprobably caused by restricted or impaired meiotic recombination as reported for R. tenuis, or alternatively by complexchromosome rearrangements or unequal condensation of homologous metaphase chromosomes. Thus, holocentricity influences the chromosomal organisation leading to differencesin the distribution patterns and condensation dynamics of centromeric and non-centromeric satDNA