Endosymbioses have profoundly impacted the evolution of life and continue to shape the ecology of a wide range of species. They give rise to new combinations of biochemical capabilities that promote innovation and diversification^1,2. Despite the many examples of known endosymbioses across the tree of life, their de novo emergence is rare and challenging to uncover in retrospect^3,4,5. Here we implant bacteria into the filamentous fungus Rhizopus microsporus to follow the fate of artificially induced endosymbioses. Whereas Escherichia coli implanted into the cytosol induced septum formation, effectively halting endosymbiogenesis, Mycetohabitans rhizoxinica was transmitted vertically to the progeny at a low frequency. Continuous positive selection on endosymbiosis mitigated initial fitness constraints by several orders of magnitude upon adaptive evolution. Phenotypic changes were underscored by the accumulation of mutations in the host as the system stabilized. The bacterium produced rhizoxin congeners in its new host, demonstrating the transfer of a metabolic function through induced endosymbiosis. Single-cell implantation thus provides a powerful experimental approach to study critical events at the onset of endosymbiogenesis and opens opportunities for synthetic approaches towards designing endosymbioses with desired traits.

内共生对生命的进化产生了深远的影响，并继续塑造着许多物种的生态学。它们产生了促进创新和多样化的生化能力的新组合^1,2。尽管在生命之树上有许多已知的内共生的例子，但它们的重新出现是罕见的，回想起来很难发现^3,4,5。在这里，我们将细菌植入丝状真菌 Rhizopus microsporus 中，以遵循人工诱导的内共生的命运。虽然植入胞质溶胶中的大肠杆菌诱导了隔膜形成，有效地阻止了内共生，但根霉菌以低频率垂直传播给后代。内共生的连续正选择在适应性进化时将初始适应度限制减轻了几个数量级。随着系统稳定，宿主中突变的积累强调了表型变化。该细菌在其新宿主中产生根瘤菌素同系物，证明代谢功能通过诱导内共生转移。因此，单细胞植入为研究内共生发生开始时的关键事件提供了一种强大的实验方法，并为设计具有所需性状的内共生的合成方法提供了机会。