Organisms harbour myriad microbes which can be parasitic or protective against harm. The costs and benefits resulting from these symbiotic relationships can be context-dependent, but the evolutionary consequences to hosts of these transitions remain unclear. Here, we mapped the Leucobacter genus across 13,715 microbiome samples (163 studies) to reveal a global distribution as a free-living microbe or a symbiont of animals and plants. We showed that across geographically distant locations (South Africa, France, Cape Verde), Leucobacter isolates vary substantially in their virulence to an associated animal host, Caenorhabditis nematodes. We further found that multiple Leucobacter sequence variants co-occur in wild Caenorhabditis spp. which combined with natural variation in virulence provides real-world potential for Leucobacter community composition to influence host fitness. We examined this by competing C. elegans genotypes that differed in susceptibility to different Leucobacter species in an evolution experiment. One Leucobacter species was found to be host-protective against another, virulent parasitic species. We tested the impact of host genetic background and Leucobacter community composition on patterns of host-based defence evolution. We found host genotypes conferring defence against the parasitic species were maintained during infection. However, when hosts were protected during coinfection, host-based defences were nearly lost from the population. Overall, our results provide insight into the role of community context in shaping host evolution during symbioses.

中文翻译：

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全球无处不在的共生体可以推动实验性宿主进化

生物体内藏有无数微生物，这些微生物可以寄生或保护免受伤害。这些共生关系产生的成本和收益可能取决于环境，但这些转变对宿主的进化后果仍不清楚。在这里，我们绘制了13,715 个微生物组样本（163 项研究）中的Leucobacter属，以揭示作为自由生活微生物或动植物共生体的全球分布。我们表明，在地理上相距较远的地区（南非、法国、佛得角），白杆菌分离株对相关动物宿主Caenorhabditis线虫的毒力差异很大。我们进一步发现在野生环境中同时存在多种白杆菌序列变异。秀丽隐杆线虫属 这与毒力的自然变异相结合，为Leucobacter群落组成影响宿主健康提供了现实世界的潜力。我们通过在进化实验中对不同Leucobacter物种的易感性不同的竞争性秀丽隐杆线虫基因型对此进行了检查。发现一种Leucobacter物种对另一种有毒的寄生物种具有宿主保护作用。我们测试了宿主遗传背景和白杆菌的影响基于宿主的防御进化模式的社区组成。我们发现在感染过程中维持了宿主基因型对寄生物种的防御。然而，当宿主在共感染期间受到保护时，基于宿主的防御几乎从种群中消失了。总的来说，我们的结果提供了对社区环境在共生过程中塑造宿主进化的作用的洞察。