Department of Microbiology and Molecular Genetics, University of Pittsburgh, Pittsburgh, PA, USA.
Department of Molecular, Cellular, and Biomedical Sciences, University of New Hampshire, Durham, NH, USA.
Mol Ecol. 2020 Jan;29(1):138-148. doi: 10.1111/mec.15307. Epub 2019 Dec 5.
Natural environments are rarely static; rather selection can fluctuate on timescales ranging from hours to centuries. However, it is unclear how adaptation to fluctuating environments differs from adaptation to constant environments at the genetic level. For bacteria, one key axis of environmental variation is selection for planktonic or biofilm modes of growth. We conducted an evolution experiment with Burkholderia cenocepacia, comparing the evolutionary dynamics of populations evolving under constant selection for either biofilm formation or planktonic growth with populations in which selection fluctuated between the two environments on a weekly basis. Populations evolved in the fluctuating environment shared many of the same genetic targets of selection as those evolved in constant biofilm selection, but were genetically distinct from the constant planktonic populations. In the fluctuating environment, mutations in the biofilm-regulating genes wspA and rpfR rose to high frequency in all replicate populations. A mutation in wspA first rose rapidly and nearly fixed during the initial biofilm phase but was subsequently displaced by a collection of rpfR mutants upon the shift to the planktonic phase. The wspA and rpfR genotypes coexisted via negative frequency-dependent selection around an equilibrium frequency that shifted between the environments. The maintenance of coexisting genotypes in the fluctuating environment was unexpected. Under temporally fluctuating environments, coexistence of two genotypes is only predicted under a narrow range of conditions, but the frequency-dependent interactions we observed provide a mechanism that can increase the likelihood of coexistence in fluctuating environments.
自然环境很少是静态的；相反，选择可以在从数小时到数百年的时间尺度上波动。然而，目前尚不清楚在遗传水平上，适应波动的环境与适应稳定的环境有何不同。对于细菌来说，环境变化的一个关键轴是选择浮游或生物膜生长模式。我们用洋葱伯克霍尔德氏菌进行了一项进化实验，比较了在恒定选择生物膜形成或浮游生长的条件下进化的种群与每周在两种环境之间波动选择的种群的进化动态。在波动环境中进化的种群与在恒定生物膜选择中进化的种群具有许多相同的选择遗传靶点，但与在恒定浮游种群中进化的种群在遗传上是不同的。在波动环境中，生物膜调节基因 wspA 和 rpfR 的突变在所有复制种群中都上升到高频率。wspA 中的突变首先在初始生物膜阶段迅速上升并几乎固定，但随后在向浮游阶段转变时被一系列 rpfR 突变体取代。wspA 和 rpfR 基因型通过负频率依赖性选择在环境之间变化的平衡点共存。在波动环境中维持共存基因型是出乎意料的。在时间波动的环境下，只有在狭窄的条件范围内才能预测两种基因型共存，但我们观察到的频率依赖性相互作用提供了一种可以增加波动环境中共存可能性的机制。
