Data from: Sanctions, partner recognition, and variation in mutualism

Mutualistic interactions can be stabilized against invasion by noncooperative individuals by putting such “cheaters” at a selective disadvantage. Selection against cheaters should eliminate genetic variation in partner quality—yet such variation is often found in natural populations. One explanation for this paradox is that mutualism outcomes are determined not only by responses to partner performance but also by partner signals. Here, we build a model of coevolution in a symbiotic mutualism, in which hosts’ ability to sanction noncooperative symbionts and recognition of symbiont signals are determined by separate loci, as are symbionts’ cooperation and expression of signals. In the model, variation persists without destabilizing the interaction, in part because coevolution of symbiont signals and host recognition is altered by the coevolution of sanctions and cooperation, and vice versa. Individual-based simulations incorporating population structure strongly corroborate these results. The dual systems of sanctions and partner recognition converge toward conditions similar to some economic models of mutualistic symbiosis, in which hosts offering the right incentives to potential symbionts can initiate symbiosis without screening for partner quality. These results predict that mutualists can maintain variation in recognition of partner signals or in the ability to sanction noncooperators without destabilizing mutualism, and they reinforce the notion that studies of mutualism should consider communication between partners as well as the exchange of benefits.互利共生相互作用可通过使这类"欺骗者（cheaters）"处于选择劣势，从而抵御非合作个体的入侵。针对欺骗者的选择本应消除伙伴质量相关的遗传变异，但此类变异在自然种群中却频繁被观测到。针对这一悖论的一种解释是：互利共生的结局不仅取决于对伙伴表现的响应，还取决于伙伴所传递的信号。 本文构建了一个共生互利体系的共演化模型，其中宿主制裁非合作共生体的能力、对共生体信号的识别能力分别由独立基因座（loci）决定，共生体的合作行为与信号表达同样由独立基因座决定。 在该模型中，遗传变异得以存续且不会破坏共生相互作用的稳定性，部分原因在于共生体信号与宿主识别的共演化过程会受到制裁机制与合作行为共演化的影响，反之亦然。纳入种群结构的基于个体的模拟实验有力佐证了上述结论。 制裁与伙伴识别的双重机制最终会收敛至与部分互利共生经济模型相似的条件：即向潜在共生体提供合理激励的宿主，无需预先筛选伙伴质量即可启动共生关系。 本研究结果表明，互利共生者可在不破坏共生稳定性的前提下，维持其对伙伴信号的识别能力或制裁非合作者的能力方面的遗传变异，同时也印证了一个核心观点：互利共生的研究不仅应关注伙伴间的收益交换，还应考虑双方之间的通信交流。