Data from: The path to re-evolve cooperation is constrained in Pseudomonas aeruginosa

Background. A common form of cooperation in bacteria is based on the secretion of beneficial metabolites, shareable as public good among cells within a group. Because cooperation can be exploited by "cheating" mutants, which contribute less or nothing to the public good, there has been great interest in understanding the conditions required for cooperation to remain evolutionarily stable. In contrast, much less is known about whether cheats, once fixed in the population, are able to revert back to cooperation when conditions change. Here, we tackle this question by subjecting experimentally evolved cheats of Pseudomonas aeruginosa, partly deficient for the production of the iron-scavenging public good pyoverdine, to conditions previously shown to favor cooperation.
Results. Following approximately 200 generations of experimental evolution, we screened 720 evolved clones for changes in their pyoverdine production levels. We found no evidence for the re-evolution of full cooperation, even in environments with increased spatial structure, and reduced costs of public good production - two conditions that have previously been shown to maintain cooperation. In contrast, we observed selection for complete abolishment of pyoverdine production. The patterns of complete trait degradation were likely driven by "cheating on cheats" in unstructured, iron-limited environments where pyoverdine is important for growth, and selection against a maladaptive trait in iron-rich environments where pyoverdine is superfluous.
Conclusions. Our study shows that the path to re-evolve public-goods cooperation can be constrained. While a limitation of the number of mutational targets potentially leading to reversion might be one reason for the observed pattern, an alternative explanation is that the selective conditions required for revertants to spread from rare are much more stringent than those needed to maintain cooperation.

研究背景。细菌常见的合作形式之一，是分泌可作为群体内细胞间共享公共物品（public good）的有益代谢物。由于合作可能被“欺骗性”突变体（cheating mutants）利用——这类突变体对公共物品的贡献更少甚至完全不贡献，学界始终高度关注合作维持进化稳定所需的条件。与之形成鲜明对比的是，学界对另一问题的认知却相对匮乏：当欺骗型菌株在种群中固定后，若环境条件发生改变，它们是否能够逆转回合作表型？本研究针对该问题展开探究：我们将经实验进化获得的、铁摄取公共物品绿脓菌素（pyoverdine）产生能力部分缺陷的铜绿假单胞菌（Pseudomonas aeruginosa）欺骗株，置于此前被证实有利于合作的环境条件中进行培养。

研究结果。经过约200代的实验进化，我们对720株进化后的克隆株的绿脓菌素产生水平进行了筛选。结果未观测到完全合作再进化的迹象，即便在空间结构增强、公共物品生产成本降低的环境中亦是如此——这两类条件此前均被证实可维持合作。与之相反，我们观察到选择压力促使绿脓菌素的产生被完全阻断。该性状彻底退化的现象，大概率由两种选择压力共同驱动：在铁限制且无空间结构的环境中（此时绿脓菌素对生长至关重要），存在“欺骗者间的互骗”现象；而在绿脓菌素过剩的富铁环境中，则存在针对适应不良性状的选择压力。

研究结论。本研究表明，公共物品合作的再进化路径可能受到约束。尽管潜在可导致表型逆转的突变靶点数量有限，或许是观测到该结果的原因之一；但另一种更具说服力的解释是：使逆转型菌株从稀有状态扩散开来所需的选择条件，远比维持合作所需的选择条件严苛得多。