The Formation and Evolution of Centromeric Satellite Repeats in Saccharum Species

Centromeres in eukaryotes are composed of tandem DNAs and retrotransposons. However, centromeric repeats exhibit considerable diversity, even among closely related species, and their origin and evolution are largely unknown. We conducted a genome-wide characterization of the centromeric sequences in sugarcane (Saccharum officinarum). Four centromeric tandem repeat sequences, So1, So103, So137 and So119, were isolated. So1 has a monomeric length of 137 bp, typical of a centromeric satellite, and has evolved four variants. However, these So1 variants had distinct centromere distributions and some were unique to an individual centromere. The distributions of the So1 variants were unexpectedly consistent among the Saccharum species that had different basic chromosome numbers or ploidy levels, thus suggesting evolutionary stability for approximately 7 million years in sugarcane. So103, So137 and So119 had unusually longer monomeric lengths that ranged from 327 to 1371 bp and lacked translational phasing on the CENH3 nucleosomes. Moreover, So103, So137 and So119 seemed to be highly similar to retrotransposons, which suggests that they originated from these mobile elements. Notably, all three repeats were flanked by direct repeats, and formed extrachromosomal circular DNAs (eccDNAs). The presence of circular molecules for these retrotransposon-derived centromeric satellites suggests an eccDNA-mediated centromeric satellite formation pathway in sugarcane.