原文链接：Ecology and evolution of facilitation among symbionts | Nature Communications

摘要

当一个物种对另一个物种的适应性产生积极影响时，就会发生促进作用，并且主要在植物等自由生活的物种中进行了研究。共生（互惠或寄生）物种或菌株之间也可能发生促进作用，但等效研究很少。为了推进促进作用对共生生态学和进化影响的综合观点，我们回顾了经验证据及其潜在机制，探索了有利于其出现的因素，并讨论了其对毒力和传播的影响。我们认为，促进作用概念可以提高对塑造共生群落及其对宿主影响的进化力量的理解。

Fig.1 Diagram showing the different types of ecological interactions. 0: no effect; –: negative effect; +: positive effect. Facilitation includes mutualistic, commensal and antagonistic interactions

Fig.2 Facilitation between symbionts can occur within or between hosts. Small blue circles: facilitating symbionts; small green circles: facilitated symbionts; big yellow circle: co-infected host; big grey circle: uninfected host; rings: entry/exit point of the host; blue dashed arrows: facilitation; black solid arrows: transition between life cycle stages of the facilitated symbiont. Traits facilitated are categorised as ‘development’ (i.e. symbiont growth or differentiation) in the within-host environment, ‘infection success’ (i.e. host entry or establishment), ‘exit from the host’ at the interface between the within- and between-host environment, and ‘transmission’ in the between-host environment. Note that the diagram does not consider the symmetry of the interaction (i.e. the effect of the facilitated symbiont on the facilitator is not represented)

Fig.3 Mechanisms underlying symbiont facilitation and parallels with free-living organisms. Blue circles: facilitating symbionts; green circles: facilitated symbionts; yellow shading: within-host environment; blue shading: between-host environment; grey or yellow circles: host; purple circles: resource; diamonds: host immune response; blue arrows: facilitation (solid: direct; dashed: indirect); black arrows: other types of effects (solid: enabled; dotted: cancelled). Traits facilitated are categorised as ‘development’ (i.e. symbiont’s growth or differentiation) in the within-host environment, ‘infection success’ (i.e. host entry or establishment) and ‘exit from the host’ at the interface between the within- and between-host environment, and ‘transmission’ in the between-host environment

Fig.4 Proposed experimental design to test the evolution of facilitation. a Experimental design required to measure evolved responses to facilitation. Small green circles: facilitated symbionts (FD); small blue circles: facilitator symbionts (FR); big grey circles: hosts. Bold coloured arrows: (co)evolving players; thin black arrows: naive players (i.e. introduced from a naive base population at each generation). Treatments: (i) evolution of the facilitator in single infections: FR(ES); (ii) evolution of the facilitator in multiple infections: FR(EM); (iii) coevolution of the facilitated and facilitator in multiple infection: FR(CM) and FD(CM), respectively; (iv) evolution of the facilitated in multiple infections: FD(EM); (v) evolution of the facilitated in single infections: FD(ES). b Example of a possible evolved response of local adaptation in which facilitated symbionts have higher fitness when assayed in the infection environment in which they evolved. (1) Facilitation; (2) adaptation to the presence of a facilitator; (3) adaptation to the presence of a coevolved facilitator; (4) cost of adaptation to the facilitator; (5) local adaptation to either a coevolved or evolved facilitator