Kiers E Toby, Rousseau Robert A, West Stuart A, Denison R Ford
Agronomy and Range Science, University of California, 1 Shields Avenue, Davis, California 95616, USA.
Nature. 2003 Sep 4;425(6953):78-81. doi: 10.1038/nature01931.
Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.
解释物种间的互利合作关系仍然是进化生物学面临的最大问题之一。为什么共生体要为宿主提供代价高昂的服务，却间接使共享同一宿主个体的竞争者受益呢？宿主对共生体表现的监测以及对“欺骗者”的制裁可能会使共生关系趋于稳定。在此我们表明，大豆会惩罚那些未能在其根瘤内固氮的根瘤菌。我们用无氮气氛（氩气：氧气）取代空气，从而阻止一种通常互利共生的根瘤菌菌株进行合作（固氮）。在三个空间尺度（整株植物、半根系和单个根瘤）上进行的一系列实验表明，迫使根瘤菌不合作（类似于欺骗行为）会使其繁殖成功率降低约50%。非侵入性监测表明，氧气供应减少可能是制裁欺骗性根瘤菌的一种机制。更普遍地说，一个或双方伙伴实施的此类制裁对于稳定广泛范围的互利共生关系可能很重要。
