Avrani Sarit, Bolotin Evgeni, Katz Sophia, Hershberg Ruth
Rachel & Menachem Mendelovitch Evolutionary Processes of Mutation & Natural Selection Research Laboratory, Department of Genetics and Developmental Biology, The Ruth and Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel.
Department of Evolutionary and Environmental Biology, The Faculty of Natural Sciences, University of Haifa, Haifa, Israel.
Mol Biol Evol. 2017 Jul 1;34(7):1758-1769. doi: 10.1093/molbev/msx118.
Many bacteria, including the model bacterium Escherichia coli can survive for years within spent media, following resource exhaustion. We carried out evolutionary experiments, followed by whole genome sequencing of hundreds of evolved clones to study the dynamics by which E. coli adapts during the first 4 months of survival under resource exhaustion. Our results reveal that bacteria evolving under resource exhaustion are subject to intense selection, manifesting in rapid mutation accumulation, enrichment in functional mutation categories and extremely convergent adaptation. In the most striking example of convergent adaptation, we found that across five independent populations adaptation to conditions of resource exhaustion occurs through mutations to the three same specific positions of the RNA polymerase core enzyme. Mutations to these three sites are strongly antagonistically pleiotropic, in that they sharply reduce exponential growth rates in fresh media. Such antagonistically pleiotropic mutations, combined with the accumulation of additional mutations, severely reduce the ability of bacteria surviving under resource exhaustion to grow exponentially in fresh media. We further demonstrate that the three positions at which these resource exhaustion mutations occur are conserved for the ancestral E. coli allele, across bacterial phyla, with the exception of nonculturable bacteria that carry the resource exhaustion allele at one of these positions, at very high frequencies. Finally, our results demonstrate that adaptation to resource exhaustion is not limited by mutational input and that bacteria are able to rapidly adapt under resource exhaustion in a temporally precise manner through allele frequency fluctuations.
包括模式细菌大肠杆菌在内的许多细菌,在资源耗尽后,能在废弃培养基中存活数年。我们进行了进化实验,随后对数百个进化克隆进行全基因组测序,以研究大肠杆菌在资源耗尽后的前4个月生存期间的适应动态。我们的结果表明,在资源耗尽条件下进化的细菌受到强烈选择,表现为快速的突变积累、功能突变类别的富集以及极其趋同的适应。在趋同适应最显著的例子中,我们发现,在五个独立种群中,对资源耗尽条件的适应是通过RNA聚合酶核心酶的三个相同特定位置发生突变实现的。这三个位点的突变具有强烈的拮抗多效性,因为它们会大幅降低在新鲜培养基中的指数生长速率。这种拮抗多效性突变,与其他突变的积累相结合,严重降低了在资源耗尽条件下存活的细菌在新鲜培养基中指数生长的能力。我们进一步证明,这些资源耗尽突变发生的三个位置,除了在其中一个位置以非常高的频率携带资源耗尽等位基因的不可培养细菌外,在细菌门中对于大肠杆菌祖先等位基因是保守的。最后,我们的结果表明,对资源耗尽的适应不受突变输入的限制,并且细菌能够通过等位基因频率波动,在资源耗尽条件下以时间精确的方式快速适应。