Host-transposon mutualism supports regeneration in planarians.

Transposons make up a significant fraction of eukaryotic genomes. They are commonly viewed as selfish elements that are detrimental to their hosts, and they are prime targets of specialized host defenses that constrain their expansion. Mutualistic interactions, in which elements co-exist and benefit each other, have so far not been found between transposable elements and eukaryote hosts. Here, we present evidence for an active transposon that confers a direct benefit to its planarian host. We find that the Ty3-like giant transposon Burro1 is an ancient element that retains its mobility. Burro1 has incorporated a host-derived anti-apoptotic protein that upregulates upon stress and improves stem cell resilience, resulting in enhanced regenerative abilities in its host. Apart from the surprising finding of a transposon's involvement in planarian regeneration, our data also uncover a true mutualistic interaction between a transposon and a eukaryote host.