Lowe Christopher D, Minter Ewan J, Cameron Duncan D, Brockhurst Michael A
Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Cornwall TR10 9FE, UK.
Department of Biology, University of York, York YO10 5DD, UK.
Curr Biol. 2016 Jan 25;26(2):207-211. doi: 10.1016/j.cub.2015.11.052. Epub 2015 Dec 31.
Endosymbiosis allows hosts to acquire new functional traits such that the combined host and endosymbiont can exploit vacant ecological niches and occupy novel environments [1, 2]; consequently, endosymbiosis affects the structure and function of ecosystems [3, 4]. However, for many endosymbioses, it is unknown whether their evolutionary basis is mutualism or exploitation [5-9]. We estimated the fitness consequences of symbiosis using the interaction between the protist host Paramecium bursaria and the algal symbiont Chlorella sp. [10]. Host fitness was strongly context dependent: whereas hosts benefited from symbiosis at high light intensity, carrying endosymbionts was costly to hosts in the dark and conferred no benefit over growing autonomously at intermediate light levels. Autonomous Chlorella densities increased monotonically with light intensity, whereas per-host symbiont load and symbiont abundance peaked at intermediate light levels and were lowest at high light intensity. This suggests that hosts controlled the costs of symbiosis by manipulating symbiont load according to light intensity. Photosynthetic efficiency was consistently lower for symbiotic compared to autonomous algae, suggesting nutritional constraints upon algae in symbiosis. At intermediate light levels, we observed the establishment of small populations of free-living algae alongside the hosts with endosymbionts, suggesting that symbionts could escape symbiosis, but only under conditions where hosts didn't benefit from symbiosis. Together, these data suggest that hosts exerted strong control over endosymbionts and that there were no conditions where this nutritional symbiosis was mutually beneficial. Our findings support theoretical predictions (e.g., [5, 9]) that controlled exploitation is an important evolutionary pathway toward stable endosymbiosis.
内共生使宿主能够获得新的功能特性，从而宿主与内共生体的组合能够开拓空缺的生态位并占据新环境[1,2]；因此，内共生影响生态系统的结构和功能[3,4]。然而，对于许多内共生现象，其进化基础是互利共生还是剥削尚不清楚[5-9]。我们利用原生生物宿主草履虫与藻类共生体小球藻之间的相互作用，估计了共生对适应性的影响[10]。宿主适应性强烈依赖于环境：虽然宿主在高光强度下从共生中受益，但在黑暗中携带内共生体对宿主来说成本高昂，并且在中等光照水平下与自主生长相比没有益处。自主生长的小球藻密度随光照强度单调增加，而每个宿主的共生体负载量和共生体丰度在中等光照水平达到峰值，在高光强度下最低。这表明宿主根据光照强度控制共生体负载量来控制共生成本。与自主生长的藻类相比，共生藻类的光合效率始终较低，这表明共生状态下藻类存在营养限制。在中等光照水平下，我们观察到在带有内共生体的宿主旁边出现了少量自由生活藻类群体，这表明共生体可以脱离共生关系，但仅在宿主无法从共生中受益的条件下。总之，这些数据表明宿主对内共生体有很强的控制作用，并且不存在这种营养共生互利的情况。我们的发现支持了理论预测（例如[5,9]），即受控剥削是通向稳定内共生的重要进化途径。
