Data from: Dynamic social behaviour in a bacterium: Pseudomonas aeruginosa partially compensates for siderophore loss to cheats

Cooperation underlies diverse phenomena including the origins of multicellular life, human behaviour in economic markets and the mechanisms by which pathogenic bacteria cause disease. Experiments with microorganisms have advanced our understanding of how, when and why cooperation evolves, but the extent to which microbial cooperation can recapitulate aspects of animal behaviour is debated. For instance, understanding the evolution of behavioural response rules (how should one individual respond to another’s decision to cooperate or defect?) is a key part of social evolution theory, but the possible existence of such rules in social microbes has not been explored. In one specific context (biparental care in animals), cooperation is maintained if individuals respond to a partner’s defection by increasing their own investment into cooperation, but not so much that this fully compensates for the defector’s lack of investment. This is termed ‘partial compensation.’ Here, I show that partial compensation for the presence of non-cooperating ‘cheats’ is also observed in a microbial social behaviour: the cooperative production of iron-scavenging siderophores by the bacterium Pseudomonas aeruginosa. A period of evolution in the presence of cheats maintains this response, whereas evolution in the absence of cheats leads to a loss of compensatory behaviour. These results demonstrate i) the remarkable flexibility of bacterial social behaviour; ii) the potential generality of partial compensation as a social response rule; and iii) the need for mathematical models to explore the evolution of response rules in multi-player social interactions.

合作支撑着诸多生命与社会现象，涵盖多细胞生命的起源、经济市场中的人类行为，以及病原细菌的致病机制。微生物相关实验极大深化了我们对合作演化的路径、时机与动因的认知，但微生物合作能否复现动物行为的诸多特征，这一议题仍存在学术争议。例如，厘清行为响应规则的演化逻辑（即个体应如何回应同伴的合作或背叛决策）是社会演化理论的核心议题之一，但社会性微生物中是否存在此类规则，迄今尚未得到探索。以动物双亲育幼这一特定情境为例：当个体通过提升自身合作投入来回应同伴的背叛行为，且该投入增幅未完全弥补背叛者的投入缺失时，合作关系得以维持，这种策略被称为‘部分补偿（partial compensation）’。本研究表明，在一种微生物社会行为——铜绿假单胞菌（Pseudomonas aeruginosa）协同产生铁载体（siderophores）以摄取环境中铁元素——中，同样存在针对不合作‘欺诈型个体（cheats）’的部分补偿行为。在欺诈型个体存在的环境下经历一段演化周期后，该响应行为得以保留；而在无欺诈型个体的环境中演化的种群，则会丧失这种补偿行为。本研究结果证实：其一，细菌社会行为具有显著的灵活性；其二，部分补偿作为社会响应规则，具备潜在的普适性；其三，亟需构建数学模型以探索多参与者社会互动中响应规则的演化过程。