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Abstract：
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.
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.
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 rarity are much more stringent than those needed to maintain cooperation.
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.
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 rarity are much more stringent than those needed to maintain cooperation.
摘要：
细菌中一种常见的合作形式是基于有益代谢物的分泌，可作为公共物品在一个组内的细胞之间共享。因为合作可以被“作弊”突变体利用，对公共利益的贡献较少或根本没有贡献，人们非常有兴趣了解合作保持进化稳定所需的条件。相比之下,人们对是否作弊知之甚少，一旦固定在人口中，当条件发生变化时，能够恢复合作。这里,我们通过实验进化的铜绿假单胞菌来解决这个问题，部分缺乏清除铁的公共物品pyroverdine的生产，以前显示的有利于合作的条件。
经过大约200代的实验进化，我们筛选了720个进化克隆的pyoverdine生产水平的变化。我们没有发现全面合作重新演变的证据，即使在空间结构增加的环境中，并降低了公共物品生产的成本-这两个条件以前已被证明可以保持合作。相比之下,我们观察到选择完全废除pyoverdine生产。完全性状退化的模式可能是由非结构化的“作弊作弊”驱动的，铁限制的环境，其中pyoverdine是重要的生长，以及在富铁环境中选择一种适应不良的性状，其中pyoverdine是多余的。
我们的研究表明，重新发展公共产品合作的路径可能会受到限制。虽然可能导致逆转的突变靶标数量的限制可能是观察到的模式的一个原因，另一种解释是，回复体从稀有性传播所需的选择条件比保持合作所需的条件严格得多。
经过大约200代的实验进化，我们筛选了720个进化克隆的pyoverdine生产水平的变化。我们没有发现全面合作重新演变的证据，即使在空间结构增加的环境中，并降低了公共物品生产的成本-这两个条件以前已被证明可以保持合作。相比之下,我们观察到选择完全废除pyoverdine生产。完全性状退化的模式可能是由非结构化的“作弊作弊”驱动的，铁限制的环境，其中pyoverdine是重要的生长，以及在富铁环境中选择一种适应不良的性状，其中pyoverdine是多余的。
我们的研究表明，重新发展公共产品合作的路径可能会受到限制。虽然可能导致逆转的突变靶标数量的限制可能是观察到的模式的一个原因，另一种解释是，回复体从稀有性传播所需的选择条件比保持合作所需的条件严格得多。
