Data from: Spontaneous nongenetic variation of group size creates cheater-free groups of social microbes

In social organisms, cheaters that gain a fitness advantage by defecting from the costs of cooperation reduce the average level of cooperation in a population. Such “cheating load” can be severe enough to cause local extinction events when cooperation is necessary for survival, but can also mediate group-level selection against cheaters across spatially structured groups that vary in cheater frequency. In cheater-laden populations, such variation could be generated by the formation of new homogeneous groups by small numbers of identical cells. Here we use the model social bacterium Myxococcus xanthus to test whether population bottlenecks inherent to the starvation-induced formation of multicellular fruiting bodies can generate cheater-free groups within an initially cheater-laden population. We first show that genetically identical fruiting bodies vary greatly in their numbers of stress-resistant spores. We further show mathematically and experimentally that this variation can include small cheater-free groups. Such non-genetic variation in group size was found to occur in a variety of M. xanthus isolates and Myxococcus species. Our results suggest that stress-induced reductions in group size may serve as a general process that repeatedly purges genetic diversity from a minority of social groups, thus recurrently generating high-relatedness social environments unburdened by cheating load.在社会生物（social organisms）中，通过逃避合作成本以获取适合度优势（fitness advantage）的欺骗者，会降低种群内合作行为的平均水平。这类“欺骗负荷（cheating load）”的严重程度足以在合作是生存必需的情况下引发局部灭绝（local extinction）事件；同时，它也能介导跨空间结构化群体（spatially structured groups）的群体层面选择，以对抗欺骗者——这些群体的欺骗者频率存在差异。在背负欺骗负荷的种群中，这类频率差异可由少量同质细胞形成新的均质群体而产生。 本研究以模式社会细菌——黄色黏球菌（Myxococcus xanthus）为实验对象，验证饥饿诱导的多细胞子实体（multicellular fruiting bodies）形成过程中固有的种群瓶颈（population bottlenecks），是否能在初始携带欺骗者的种群中催生无欺骗者群体。我们首先证实，遗传背景一致的子实体在抗逆孢子（stress-resistant spores）的数量上存在显著差异。随后我们通过数学建模与实验证明，这类差异可包含小型无欺骗者群体。这种群体规模的非遗传变异（non-genetic variation）在多种黄色黏球菌分离株及黏球菌（Myxococcus）物种中均被观测到。我们的研究结果表明，应激诱导的群体规模缩减或许是一种通用过程，可反复从少数社会群体中清除遗传多样性，从而持续生成无欺骗负荷负担的高亲缘性社会环境。