Data from: How obstacles perturb population fronts and alter their genetic structure

As populations spread into new territory, environmental heterogeneities can shape the population front and genetic composition. We focus here on the effects of an important building block of heterogeneous environments, isolated obstacles. With a combination of experiments, theory, and simulation, we show how isolated obstacles both create long-lived distortions of the front shape and amplify the effect of genetic drift. A system of bacteriophage T7 spreading on a spatially heterogeneous Escherichia coli lawn serves as an experimental model system to study population expansions. Using an inkjet printer, we create well-defined replicates of the lawn and quantitatively study the population expansion of phage T7. The transient perturbations of the population front found in the experiments are well described by a model in which the front moves with constant speed. Independent of the precise details of the expansion, we show that obstacles create a kink in the front that persists over large distances and is insensitive to the details of the obstacle’s shape. The small deviations between experimental findings and the predictions of the constant speed model can be understood with a more general reaction-diffusion model, which reduces to the constant speed model when the obstacle size is large compared to the front width. Using this framework, we demonstrate that frontier genotypes just grazing the side of an isolated obstacle increase in abundance, a phenomenon we call ‘geometry-enhanced genetic drift’, complementary to the founder effect associated with spatial bottlenecks. Bacterial range expansions around nutrient-poor barriers and stochastic simulations confirm this prediction. The effect of the obstacle on the genealogy of individuals at the front is characterized by simulations and rationalized using the constant speed model. Lastly, we consider the effect of two obstacles on front shape and genetic composition of the population illuminating the effects expected from complex environments with many obstacles.

当种群向新领地扩散时，环境异质性可塑造种群前沿（population front）与遗传组成。本文聚焦异质环境中的重要构成单元——孤立障碍物的影响。我们结合实验、理论与模拟手段，揭示了孤立障碍物如何引发种群前沿形态的长期畸变，并放大遗传漂变的效应。我们以在空间异质性大肠杆菌（Escherichia coli）菌苔上铺展的噬菌体T7（bacteriophage T7）种群扩张体系，作为研究种群扩张的实验模型系统。借助喷墨打印机，我们制备了重复性良好的标准化菌苔复制品，对噬菌体T7的种群扩张过程进行定量研究。实验中观测到的种群前沿瞬态扰动，可通过前沿以恒定速度移动的模型得到良好拟合。无论种群扩张的具体细节如何，我们均证明：障碍物会在种群前沿形成一处可长距离持续存在的扭结，且该扭结不受障碍物形状细节的影响。实验结果与恒定速度模型预测值之间的细微偏差，可通过更通用的反应扩散模型（reaction-diffusion model）得到解释；当障碍物尺寸远大于前沿宽度时，该模型可简化为恒定速度模型。基于此分析框架，我们证实：恰好擦过孤立障碍物边缘的前沿基因型丰度会升高，我们将这一现象命名为“几何增强型遗传漂变”，其与空间瓶颈相关的奠基者效应（founder effect）形成互补。围绕营养匮乏屏障开展的细菌种群扩张实验与随机模拟验证了这一预测。障碍物对种群前沿个体谱系的影响可通过模拟表征，并借助恒定速度模型得到合理解释。最后，我们探究了双障碍物对种群前沿形态与种群遗传组成的影响，以此阐明多障碍物复杂环境中可观测到的预期效应。