New Insights into Centromere Organization and Evolution from the White-cheeked Gibbon and Marmoset.

The evolutionary history of alpha-satellite DNA, the major component of primate centromeres, is hardly defined because of the difficulty in its sequence assembly and its rapid evolution when compared to most genomic sequences. By using several approaches, we have cloned, sequenced and characterized {alpha}-satellite sequences from two species representing critical nodes in the primate phylogeny: the white-cheeked gibbon, a lesser ape and marmoset, a New World monkey. Sequence analyses demonstrate that white-cheeked gibbon and marmoset {alpha}-satellite sequences are formed by units of ~171 bp and ~342 bp, respectively, and they both lack high-order structure found in humans and great apes. FISH characterization shows a broad dispersal of alpha-satellite in the white-cheeked gibbon genome including centromeric, telomeric and chromosomal interstitial localizations. On the other side centromeres in marmoset appear organized in highly divergent dimers roughly of 342bp that show a similarity between monomers much lower than previously reported dimers thus representing an ancient dimeric structure. All these data shed light on the evolution of the centromeric sequences in Primates. Our results suggest radical differences in the structure, organization and evolution of {alpha}-satellite DNA among different primate species, supporting the notion that i) all the centromeric sequence in Primates evolved by genomic amplification, unequal crossover and sequence homogenization using a 171bp monomer as basic seeding unit and ii) centromeric function is linked to relatively short repeated elements, more than higher-order structure. Moreover, our data indicate that complex higher-order repeat structures are a peculiarity of the hominid lineage, showing the more complex organization in human.