Nonselective bottlenecks control the divergence and diversification of phase-variable bacterial populations

Phase variation occurs in many pathogenic and commensal bacteria and is a major generator of genetic variability. A putative advantage of phase variation is to counter reductions in variability imposed by nonselective bottlenecks during transmission. Genomes of Campylobacter jejuni, a widespread food-borne pathogen, contain multiple phase-variable loci whose rapid, stochastic variation is generated by hypermutable simple sequence repeat tracts. These loci can occupy a vast number of combinatorial expression states (phasotypes) enabling populations to rapidly access phenotypic diversity. The imposition of nonselective bottlenecks can perturb the relative frequencies of phasotypes, changing both within-population diversity and divergence from the initial population. Using both in vitro testing of C. jejuni populations and a simple stochastic simulation of phasotype change, we observed that single-cell bottlenecks produce output populations of low diversity but with bimodal patterns of eit...相变异（phase variation）广泛存在于多种致病菌与共生菌中，是遗传变异的重要来源之一。相变异的一个推定优势在于，可抵消传播过程中非选择性瓶颈所带来的变异水平下降。作为一类广泛分布的食源性病原菌，空肠弯曲菌（Campylobacter jejuni）的基因组中存在多个相变异可变位点，其快速随机变异由高突变性简单序列重复片段（hypermutable simple sequence repeat tracts）驱动产生。这些位点可形成海量的组合表达状态（相型，phasotypes），使菌群群体能够快速获取表型多样性。引入非选择性瓶颈会扰动相型的相对频率，进而同时改变种群内的多样性水平及与初始种群的分化程度。本研究通过对空肠弯曲菌菌群开展体外实验，并结合针对相型变化的简单随机模拟，发现单细胞瓶颈可产生低多样性的输出菌群，但呈现出bimodal patterns of eit...