Data from: Microbial expansion-collision dynamics promote cooperation and coexistence on surfaces

Microbes colonizing a surface often experience colony growth dynamics characterized by an initial phase of spatial clonal expansion followed by collision between neighboring colonies to form potentially genetically heterogeneous boundaries. For species with life cycles consisting of repeated surface colonization and dispersal, these spatially-explicit “expansion-collision dynamics” generate periodic transitions between two distinct selective regimes, “expansion competition” and “boundary competition”, each one favoring a different growth strategy. We hypothesized that this dynamic could promote stable coexistence of expansion- and boundary-competition specialists by generating time-varying, negative frequency-dependent selection that insulates both types from extinction. We tested this experimentally in budding yeast by competing an exo-enzyme secreting “cooperator” strain (expansion-competition specialists) against non-secreting “defectors” (boundary-competition specialists). As predicted, we observed cooperator-defector coexistence or cooperator dominance with expansion-collision dynamics, but only defector dominance otherwise. Also as predicted, the steady-state frequency of cooperators was determined by colonization density (the average initial cell-cell distance) and cost of cooperation. Lattice-based spatial simulations give good qualitative agreement with experiments, supporting our hypothesis that expansion-collision dynamics with costly public goods production is sufficient to generate stable cooperator-defector coexistence. This mechanism may be important for maintaining public-goods cooperation-and-conflict in microbial pioneer species living on surfaces.

定殖于表面的微生物通常会呈现出菌落生长动态特征：初始阶段发生空间克隆扩增，随后相邻菌落相互碰撞，形成可能具有遗传异质性的边界区域。对于那些生命周期包含反复表面定殖与扩散过程的物种而言，这类空间显式（spatially-explicit）的“扩增-碰撞动态”会在两种截然不同的选择制度——“扩增竞争”与“边界竞争”——之间产生周期性转换，二者分别青睐不同的生长策略。我们提出假说：该动态过程可通过产生随时间变化的负频率依赖选择（negative frequency-dependent selection），使两类专化菌株均免于灭绝，从而促进扩增竞争专化菌株与边界竞争专化菌株的稳定共存。我们以出芽酵母（budding yeast）为实验对象，将分泌胞外酶的“合作者”菌株（扩增竞争专化菌株）与不分泌酶的“欺骗者”菌株（边界竞争专化菌株）进行竞争实验，以此验证上述假说。正如预期，在存在扩增-碰撞动态的条件下，我们观测到了合作者与欺骗者的稳定共存，或是合作者占据优势；而在缺乏该动态的对照组中，则仅出现欺骗者占据优势的结果。同样符合预期的是，合作者的稳态频率由定殖密度（初始细胞间平均距离）与合作成本共同决定。基于晶格的空间模拟结果与实验结果吻合度较高，定性上一致，验证了我们的假说：带有代价性公共物品（public goods）生产的扩增-碰撞动态足以促成合作者与欺骗者的稳定共存。该机制对于维持定殖于表面的微生物拓殖物种中的公共物品合作与冲突平衡具有重要意义。