Tracing evolutionary processes that lead to fixation of genomic variation in wild bacterial populations is a prime challenge in molecular evolution. In particular, the relative contribution of Horizontal Gene Transfer (HGT) versus de novo mutations during adaptation to a new environment is poorly understood. To gain a better understanding of the dynamics of HGT and its effect on adaptation, we subjected several populations of competent Bacillus subtilis to a serial dilution evolution on a high salt containing medium, either with or without foreign DNA from diverse pre-adapted or naturally salt tolerant species. Following 504 generations of evolution, all populations improved growth yield on the medium. Sequencing of evolved populations revealed extensive acquisition of foreign DNA from close Bacillus donors but not from more remote donors. HGT occurred in bursts, whereby a single bacterial cell appears to have acquired dozens of fragments at once. In the largest burst, close to 2% of the genome has been replaced by HGT. Acquired segments tend to be clustered in integration "hot spots". Other than HGT, genomes also acquired spontaneous mutations. Many of these mutations occurred within, and seem to alter, the sequence of flagellar proteins. Finally, we show that while some HGT fragments could be neutral, others are adaptive and accelerate evolution.

中文翻译：

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枯草芽孢杆菌的实验进化揭示了水平基因转移的进化动力学，并提出了适应性和中性效应。


追踪导致野生细菌群体基因组变异固定的进化过程是分子进化中的主要挑战。特别是，在适应新环境过程中水平基因转移（HGT）与从头突变的相对贡献知之甚少。为了更好地了解 HGT 的动态及其对适应的影响，我们将几个感受态枯草芽孢杆菌群体在高盐培养基上进行系列稀释进化，其中含有或不含有来自不同预适应或天然盐的外源 DNA耐受的物种。经过 504 代的进化，所有种群在培养基上的生长产量均有所提高。对进化群体的测序表明，从邻近的芽孢杆菌捐赠者那里大量获得了外源DNA，但不是从更远的捐赠者那里获得的。 HGT 以爆发的方式发生，单个细菌细胞似乎同时获得了数十个片段。在最大规模的爆发中，接近 2% 的基因组已被 HGT 取代。获得的细分市场往往聚集在整合“热点”中。除了 HGT 之外，基因组还获得了自发突变。其中许多突变发生在鞭毛蛋白的序列内，并且似乎改变了鞭毛蛋白的序列。最后，我们表明，虽然一些 HGT 片段可能是中性的，但其他片段则具有适应性并加速进化。