Harvard University, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2010 Sep 7;107(36):15712-6. doi: 10.1073/pnas.1005294107. Epub 2010 Aug 23.
Although mutualisms are common in all ecological communities and have played key roles in the diversification of life, our current understanding of the evolution of cooperation applies mostly to social behavior within a species. A central question is whether mutualisms persist because hosts have evolved costly punishment of cheaters. Here, we use the economic theory of employment contracts to formulate and distinguish between two mechanisms that have been proposed to prevent cheating in host-symbiont mutualisms, partner fidelity feedback (PFF) and host sanctions (HS). Under PFF, positive feedback between host fitness and symbiont fitness is sufficient to prevent cheating; in contrast, HS posits the necessity of costly punishment to maintain mutualism. A coevolutionary model of mutualism finds that HS are unlikely to evolve de novo, and published data on legume-rhizobia and yucca-moth mutualisms are consistent with PFF and not with HS. Thus, in systems considered to be textbook cases of HS, we find poor support for the theory that hosts have evolved to punish cheating symbionts; instead, we show that even horizontally transmitted mutualisms can be stabilized via PFF. PFF theory may place previously underappreciated constraints on the evolution of mutualism and explain why punishment is far from ubiquitous in nature.
虽然互利共生在所有生态群落中都很常见，并且在生命多样化中发挥了关键作用，但我们目前对合作进化的理解主要适用于物种内的社会行为。一个核心问题是，互利共生是否能够持续存在，因为宿主已经进化出了对欺骗者的昂贵惩罚。在这里，我们使用就业合同的经济理论来制定和区分两种机制，这两种机制被提议用于防止宿主-共生体互利共生中的欺骗行为，即伙伴保真反馈（PFF）和宿主制裁（HS）。在 PFF 下，宿主适应性和共生体适应性之间的正反馈足以防止欺骗；相比之下，HS 假定维持互利共生需要昂贵的惩罚。互利共生的共进化模型发现，HS 不太可能从头开始进化，并且关于豆科植物-根瘤菌和丝兰蛾-丝兰互利共生的已发表数据与 PFF 一致，而与 HS 不一致。因此，在被认为是 HS 典型案例的系统中，我们发现宿主已经进化出惩罚欺骗共生体的理论的证据不足；相反，我们表明，即使是水平传播的互利共生也可以通过 PFF 来稳定。PFF 理论可能对互利共生的进化施加了以前未被充分认识的限制，并解释了为什么惩罚在自然界中远非普遍存在。
