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（environmental heterogeneity）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（isolated obstacle）的影响。通过结合实验、理论与模拟手段，我们揭示了孤立障碍物如何长期扭曲种群前沿形态，并放大遗传漂变（genetic drift）的效应。以铺展于空间异质性大肠杆菌平板（Escherichia coli lawn）上的噬菌体T7（bacteriophage T7）种群扩张为实验模型系统，我们利用喷墨打印机制备了重复性良好的平行样本，并定量研究了噬菌体T7的种群扩张过程。实验中观测到的种群前沿瞬时扰动，可通过以恒定速度移动的前沿模型得到良好拟合。无论种群扩张的具体细节如何，障碍物均可在种群前沿形成可长距离维持的折曲，且该结构不受障碍物形状细节的影响。恒定速度模型与实验结果间的微小偏差，可通过更通用的反应扩散模型（reaction-diffusion model）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。基于此框架，我们证明仅擦过孤立障碍物侧面的前沿基因型丰度会提升，本文将这一现象命名为"几何增强型遗传漂变（geometry-enhanced genetic drift）"，其与空间瓶颈相关的奠基者效应（founder effect）形成互补。针对营养匮乏障碍物的细菌种群扩张实验与随机模拟，均证实了这一预测。我们通过模拟表征了障碍物对前沿个体基因谱系的影响，并借助恒定速度模型对其进行了合理化阐释。最后，本文还探讨了双障碍物对种群前沿形态与种群遗传组成的影响，为理解多障碍物复杂环境中的效应提供了参考。