Department of Zoology, University of Oxford, Oxford OX4 1YL, UK.
Am Nat. 2012 Sep;180(3):296-305. doi: 10.1086/667193. Epub 2012 Jul 25.
Microbes produce many molecules that are important for their growth and development, and the exploitation of these secretions by nonproducers has recently become an important paradigm in microbial social evolution. Although the production of these public-goods molecules has been studied intensely, little is known of how the benefits accrued and the costs incurred depend on the quantity of public-goods molecules produced. We focus here on the relationship between the shape of the benefit curve and cellular density, using a model assuming three types of benefit functions: diminishing, accelerating, and sigmoidal (accelerating and then diminishing). We classify the latter two as being synergistic and argue that sigmoidal curves are common in microbial systems. Synergistic benefit curves interact with group sizes to give very different expected evolutionary dynamics. In particular, we show that whether and to what extent microbes evolve to produce public goods depends strongly on group size. We show that synergy can create an "evolutionary trap" that can stymie the establishment and maintenance of cooperation. By allowing density-dependent regulation of production (quorum sensing), we show how this trap may be avoided. We discuss the implications of our results on experimental design.
微生物产生许多对其生长和发育很重要的分子，而非生产者对这些分泌物的开发利用最近已成为微生物社会进化中的一个重要范例。尽管这些公共利益分子的产生已经被深入研究，但对于收益和成本如何取决于公共利益分子的产生数量却知之甚少。我们在这里重点研究收益曲线的形状与细胞密度之间的关系，使用了一个假设三种收益函数的模型：递减、加速和 S 形（先加速后递减）。我们将后两种归为协同作用，并认为 S 形曲线在微生物系统中很常见。协同作用的收益曲线与群体规模相互作用，会产生非常不同的预期进化动态。特别是，我们表明，微生物是否以及在何种程度上进化为产生公共利益品强烈取决于群体规模。我们表明，协同作用可能会造成一种“进化陷阱”，从而阻碍合作的建立和维持。通过允许生产的密度依赖性调节（群体感应），我们展示了如何避免这种陷阱。我们讨论了我们的研究结果对实验设计的影响。
