Institute of Integrative Biology, ETH Zürich, Zürich, Switzerland.
Microbial Evolutionary Genomics Unit, Institut Pasteur, Paris, France.
Mol Ecol. 2018 Aug;27(15):3146-3158. doi: 10.1111/mec.14773. Epub 2018 Jul 6.
Genetically similar cells of the soil bacterium Myxococcus xanthus cooperate at multiple social behaviours, including motility and multicellular development. Another social interaction in this species is outer membrane exchange (OME), a behaviour of unknown primary benefit in which cells displaying closely related variants of the outer membrane protein TraA transiently fuse and exchange membrane contents. Functionally incompatible TraA variants do not mediate OME, which led to the proposal that TraA incompatibilities determine patterns of intercellular cooperation in nature, but how this might occur remains unclear. Using natural isolates from a centimetre-scale patch of soil, we analyse patterns of TraA diversity and ask whether relatedness at TraA is causally related to patterns of kin discrimination in the form of both colony-merger incompatibilities (CMIs) and interstrain antagonisms. A large proportion of TraA functional diversity documented among global isolates is predicted to be contained within this cm-scale population. We find evidence of balancing selection on the highly variable PA14-portion of TraA and extensive transfer of traA alleles across genomic backgrounds. CMIs are shown to be common among strains identical at TraA, suggesting that CMIs are not generally caused by TraA dissimilarity. Finally, it has been proposed that interstrain antagonisms might be caused by OME-mediated toxin transfer. However, we predict that most strain pairs previously shown to exhibit strong antagonisms are incapable of OME due to TraA dissimilarity. Overall, our results suggest that most documented patterns of kin discrimination in a natural population of M. xanthus are not causally related to the TraA sequences of interactants.
土壤细菌粘球菌的遗传相似细胞在多种社会行为中合作，包括运动和多细胞发育。该物种的另一种社会相互作用是外膜交换 (OME)，这是一种未知主要益处的行为，其中显示出紧密相关的外膜蛋白 TraA 变体的细胞短暂融合并交换膜内容物。功能上不兼容的 TraA 变体不能介导 OME，这导致 TraA 不兼容性决定了自然界中细胞间合作的模式，但这是如何发生的尚不清楚。使用来自土壤一厘米大小斑块的天然分离物，我们分析了 TraA 多样性的模式，并询问 TraA 之间的亲缘关系是否与以菌落融合不兼容 (CMIs) 和菌株间拮抗作用形式的细胞间合作模式有关。在全球分离株中记录的大量 TraA 功能多样性预计包含在这个厘米尺度的种群中。我们发现高度可变的 TraA 的 PA14 部分存在平衡选择的证据，并广泛转移 traA 等位基因跨越基因组背景。TraA 相同的菌株之间存在常见的 CMIs，这表明 CMIs 通常不是由 TraA 不同引起的。最后，有人提出菌株间拮抗作用可能是由 OME 介导的毒素转移引起的。然而，我们预测，由于 TraA 不同，以前显示出强烈拮抗作用的大多数菌株对之间不能进行 OME。总的来说，我们的研究结果表明，在粘球菌的自然种群中，大多数记录的亲缘关系识别模式与相互作用的 TraA 序列没有因果关系。
