Phylogenetic evidence suggests that the invasion and proliferation of retroelements, selfish mobile genetic elements that copy and paste themselves within a host genome, was one of the early evolutionary events in the emergence of eukaryotes. Here we test the effects of this event by determining the pressures retroelements exert on simple genomes. We transferred two retroelements, human LINE-1 and the bacterial group II intron Ll.LtrB, into bacteria, and find that both are functional and detrimental to growth. We find, surprisingly, that retroelement lethality and proliferation are enhanced by the ability to perform eukaryotic-like nonhomologous end-joining (NHEJ) DNA repair. We show that the only stable evolutionary consequence in simple cells is maintenance of retroelements in low numbers, suggesting how retrotransposition rates and costs in early eukaryotes could have been constrained to allow proliferation. Our results suggest that the interplay between NHEJ and retroelements may have played a fundamental and previously unappreciated role in facilitating the proliferation of retroelements, elements of which became the ancestors of the spliceosome components in eukaryotes.

系统发育学证据表明，逆转录元件的入侵和增殖是自发的可移动的遗传元件，可在宿主基因组中复制并粘贴自身，是真核生物出现的早期进化事件之一。在这里，我们通过确定逆转录元素对简单基因组施加的压力来测试此事件的影响。我们将两个反转录因子，人LINE-1和细菌II组内含子Ll.LtrB转移到细菌中，发现它们都具有功能性且不利于生长。我们惊奇地发现，通过进行真核样非同源末端连接（NHEJ）DNA修复的能力可以增强逆转录致死力和增殖。我们表明，简单细胞中唯一稳定的进化结果是维持少量的逆转录元素，这表明如何限制早期真核生物的逆转座率和成本以使其扩散。我们的研究结果表明，NHEJ和逆转录元件之间的相互作用可能在促进逆转录元件的增殖中起了根本性的作用，而以前却未被重视，而逆转录元件中的元素已成为真核生物中剪接体成分的祖先。