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.

转座子（Transposon）在真核生物基因组中占比可观。长期以来，这类遗传元件被视作损害宿主的自私基因载体，同时也是宿主专属防御系统的核心靶标，这类防御机制会抑制转座子的扩增。尽管互利共生关系在生物学其他分支中极为普遍，但此前学界普遍认为转座因子（Transposable Element）与真核生物之间并不存在此类相互作用。本研究首次提供证据，证明一种具有活性的转座子可直接为其真核宿主带来益处。我们发现，类Ty3巨型转座子Burro1是一种古老的转座因子，至今仍保留转座活性，并在真涡虫基因组中发生了活跃扩增。Burro1编码一种源自宿主的功能性抗细胞凋亡蛋白，该蛋白可在应激条件下上调表达，提升干细胞的存活率。借此机制，该转座子增强了干细胞的抗逆性，进而提升了携带该转座子的真涡虫的再生能力。除了转座子参与真涡虫再生这一突破性发现之外，本研究的数据还证实了转座子与宿主之间确实存在互利共生相互作用。这一发现为探究宿主针对转座子的适应性防御机制，以及转座因子对真核生物基因组的影响提供了全新的研究视角。