"Great fleas have little fleas upon their backs to bite 'em, and little fleas have lesser fleas, and so ad infinitum"

Transposable Elements (TEs) are fragments of DNA that can jump from one genome position to another producing extra copies of themselves in the process. In a recent issue of Science, Josefa Gonzalez and Dmitri Petrov write a perspective on a paper by Yang et al which showed how some TEs manage to dispense with almost all of their sequences and still remain extremely prolific. TEs generally encode among other genes proteins that promote their mobility, either a reverse transcriptase or a transposase and parasitize the key cellular functions. Interestingly, such TEs are themselves often parasitized. These parasites of parasites -- less judgmentally called non-autonomous TEs -- contain key recognition sequence required for mobility but dispense with making the protein products required for transposition. A spectacularly successful type of non-autonomous TEs, called MITEs, has been discovered fairly recently in plants. MITEs are present in many thousand copies in many plant genomes but because they are so small (~100- 500 bp) and encode no proteins it was hard to understand how they move. We now have a very good model but still have plenty of unresolved puzzles. For more details read our Perspective and the Yang et al. paper.