Biosciences, University of Exeter, Cornwall, UK.
J Evol Biol. 2018 Sep;31(9):1330-1339. doi: 10.1111/jeb.13307. Epub 2018 Jul 2.
Cooperation can be maintained if cooperative behaviours are preferentially directed towards other cooperative individuals. Tag-based cooperation (greenbeards) - where cooperation benefits individuals with the same tag as the actor - is one way to achieve this. Tag-based cooperation can be exploited by individuals who maintain the specific tag but do not cooperate, and selection to escape this exploitation can result in the evolution of tag diversity. We tested key predictions crucial for the evolution of cheat-mediated tag diversity using the production of iron-scavenging pyoverdine by the opportunistic pathogen, Pseduomonas aeruginosa as a model system. Using two strains that produce different pyoverdine types and their respective cheats, we show that cheats outcompete their homologous pyoverdine producer, but are outcompeted by the heterologous producer in well-mixed environments. As a consequence, co-inoculating two types of pyoverdine producer and one type of pyoverdine cheat resulted in the pyoverdine type whose cheat was not present having a large fitness advantage. Theory suggests that in such interactions, cheats can maintain tag diversity in spatially structured environments, but that tag-based cooperation will be lost in well-mixed populations, regardless of tag diversity. We saw that when all pyoverdine producers and cheats were co-inoculated in well-mixed environments, both types of pyoverdine producers were outcompeted, whereas spatial structure (agar plates and compost microcosms), rather than maintaining diversity, resulted in the domination of one pyoverdine producer. These results suggest cheats may play a more limited role in the evolution of pyoverdine diversity than predicted.
如果合作行为优先指向其他合作个体，那么合作就可以维持。基于标签的合作（greenbeards）——其中合作使与行为者具有相同标签的个体受益——是实现这一目标的一种方式。那些维持特定标签但不合作的个体可以利用基于标签的合作，而逃避这种剥削的选择可以导致标签多样性的进化。我们使用机会性病原体铜绿假单胞菌生产铁掠夺素 pyoverdine 作为模型系统，测试了对于欺骗介导的标签多样性进化至关重要的关键预测。使用产生不同 pyoverdine 类型的两种菌株及其各自的欺骗者，我们表明欺骗者会胜过与其同源的 pyoverdine 生产者，但在充分混合的环境中会被异源生产者胜过。因此，同时接种两种类型的 pyoverdine 生产者和一种类型的 pyoverdine 欺骗者会导致不存在欺骗者的 pyoverdine 类型具有很大的适应优势。理论表明，在这种相互作用中，欺骗者可以在空间结构环境中维持标签多样性，但无论标签多样性如何，基于标签的合作将在充分混合的群体中丧失。我们发现，当所有 pyoverdine 生产者和欺骗者在充分混合的环境中同时接种时，两种类型的 pyoverdine 生产者都被淘汰，而空间结构（琼脂平板和堆肥微宇宙）不仅维持了多样性，还导致了一种 pyoverdine 生产者的主导地位。这些结果表明，欺骗者在 pyoverdine 多样性进化中的作用可能比预期的更为有限。
