Endosymbiosis, an interaction between two species where one lives within the other, has evolved multiple times independently, but the underlying mechanisms remain unclear. Evolutionary theory suggests that for an endosymbiotic interaction to remain stable over time, births of both partners should be higher than their deaths in symbiosis and deaths of both partners should be higher than their births when living independently. However, experimentally measuring this can be difficult and conclusions tend to focus on the host. Using a ciliate–algal system (Paramecium bursaria host and Chlorella endosymbionts), we estimated the benefits and costs of endosymbiosis for both organisms using fitness measurements in different biotic environments to test under which environmental conditions the net effects of the interaction were positive for both partners. We found that the net effects of harbouring endosymbionts were positive for the ciliate hosts as it allowed them to survive in conditions of low-quality bacteria food. The algae benefitted by being endosymbiotic when predators such as the hosts were present, but the net effects were dependent on the total density of hosts, decreasing as hosts densities increased. Overall, we show that including context-dependency of endosymbiosis is essential in understanding how these interactions have evolved.

中文翻译：

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纤毛虫-藻类系统中内共生相互作用的上下文相关成本和收益

内共生是两个物种之间的一种相互作用，其中一个物种生活在另一个物种中，它已经独立进化了多次，但潜在的机制仍不清楚。进化论表明，为了使内共生相互作用随着时间的推移保持稳定，双方的出生率应该高于共生时的死亡率，而双方的死亡率应该高于独立生活时的出生率。然而，通过实验测量这一点可能很困难，而且结论往往集中在宿主身上。使用纤毛虫-藻类系统（Paramecium bursaria宿主和Chlorella内共生体），我们使用不同生物环境中的适应性测量来估计内共生对两种生物体的益处和成本，以测试在哪些环境条件下相互作用的净效应对两个伙伴都是积极的。我们发现，携带内共生体的净效应对纤毛虫宿主是积极的，因为它使它们能够在低质量细菌食物的条件下生存。当宿主等捕食者存在时，藻类会因内共生而受益，但净效应取决于宿主的总密度，随着宿主密度的增加而降低。总的来说，我们表明，包括内共生的环境依赖性对于理解这些相互作用是如何进化的至关重要。