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Background  We recently discovered two composite long terminal repeat (LTR)-retrotransposon-like elements which we named DA (~300 kb) and Xiao (~30 kb), meaning big and small in Chinese respectively. Xiao and DA (three types of DA identified) were found to have been derived from several donor sites and have spread to 30 loci in the human genome, totaling to 5 Mb. Our bioinformatics analyses with the released human, chimp, rhesus macaque, orangutan, and marmoset genomic sequences indicate that DA and Xiao emerged ~25 million years (Myr) ago.    Results  To better understand the evolution of these two complex elements, we investigated various internal junctions of DA and Xiao as well as orthologous genomic sites of the 30 DA/Xiao loci in non-human primates including great apes, lesser apes, Old World monkeys, New World monkeys, and a prosimian. We found that Xiao and type I DA first emerged in the genome between 25 and 18 Myr ago, whereas type II and Type III DAs emerged between 14 and 7 Myr ago. Xiao and DA were most active in great apes, with their amplification peaking during 25-14 and 14-7 Myr ago, respectively. Neither DA nor Xiao seem to have been active in the human and chimp genomes during last 6 Myr.    Conclusion  The study has led to a more accurate age determination of the DA and Xiao elements than our previous bioinformatics analyses, and indicates that the amplification activity of the elements coincided with that of group I HERV-Es during evolution. It has also illustrated an evolutionary path with stepwise structural changes for the elements during past 25 Myr, and in doing so has shed more light on these two intriguing and complex elements that have reshaped our genome.

Stepwise evolution of two giant composite LTR-retrotransposon-like elements DA and Xiao