Are retrotransposons long-term hitchhikers?

Transposable elements represent a large fraction of the genomes of eukaryotes, and yet we know little of their origins or stability. Striking examples of cross-species transfer have been discovered among mariner elements1 (transposable elements that are widespread in insects and other animals), confirming the impression that horizontal transfers are essential to the long-term success of transposable elements. We show that R1 and R2, two distantly related non-long-terminal-repeat retrotransposons which insert at specific sites 74 base pairs apart in 28S ribosomal RNA genes2, have been maintained by vertical transmission since the origin of the phylum Arthropoda, that is, for at least 500 million years.