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. • Burro1 is a composite transposon that escaped inactivation for 100 million years • Burro1 captured an anti-apoptotic host gene ( B-iap ) that yields a functional protein • Burro1 (including B-iap ) transcription is upregulated during stress • Stress-induced B-IAP enhances stem cell survival and planarian regeneration Lee et al. find that the planarian transposon Burro1 bolsters stem cell resilience by producing a functional host-derived caspase inhibitor during physiological stress. By suppressing apoptosis, active Burro1 elements enhance the regenerative capacity of planarians and have sustained this mutualistic relationship for over 100 million years.