Host-transposon mutualism supports regeneration in planarians
Transposons make up significant fractions 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, while common in other realms of biology, have so far appeared nonexistent between transposable elements and eukaryotes. Here, we present evidence for an active transposon that confers a direct benefit to its eukaryotic host. We find that the Ty3-like giant transposon Burro1 is an ancient transposon that retains its mobility and has actively expanded in the planarian genome. Burro1 encodes a functional copy of a host-derived anti-poptotic protein that upregulates upon stress and improves stem cell survival. Hereby it enhances the resilience of the stem cells, resulting in enhanced regenerative abilities of the planarians that host this transposon. Apart from the surprising finding of a transposon involvement in planarian regeneration, our data also uncover the existence of mutualistic interactions between transposons and their hosts. This provides a new lens through which to investigate the adaptive host defenses against transposons, and the impact of transposable elements on eukaryote genomes.