Evolutionary history of alpha satellite DNA repeats dispersed within human genome euchromatin

Major human alpha satellite DNA repeats are preferentially assembled within (peri)centromeric regions but are also dispersed within euchromatin in the form of clustered or short single repeat arrays. To study the evolutionary history of single euchromatic human alpha satellite repeats (AR) we analysed their orthologous loci across the primate genomes. The continuous insertion of euchromatic ARs throughout the evolutionary history of primates starting with the ancestors of Simiformes (45-60 Myr ago) and continuing up to the ancestors of Homo is revealed. Once inserted, the euchromatic ARs were stably transmitted to the descendant species, some exhibiting copy number variation, while their sequence divergence followed the species phylogeny. Many euchromatic ARs have sequence characteristics of (peri)centromeric alpha repeats suggesting heterochromatin as a source of dispersed euchromatic ARs. The majority of euchromatic ARs are inserted in the vicinity of other repetitive elements such as L1, Alu and ERV, or are embedded within them. Irrespective of the insertion context, each AR insertion seems to be unique and once inserted, ARs do not seem to be subsequently spread to new genomic locations. In spite of association with (retro)transposable elements there is no indication that such elements play a role in ARs proliferation. The presence of short duplications at most of ARs insertion sites suggests site directed recombination between homologous motifs in ARs and in the target genomic sequence, probably mediated by extrachromosomal circular DNA, as a mechanism of spreading within euchromatin.