Department of Ecological Science, VU University, 1081 HV Amsterdam, Netherlands.
Am Nat. 2012 May;179(5):E133-46. doi: 10.1086/665032. Epub 2012 Mar 29.
Explaining mutualistic cooperation between species remains a major challenge for evolutionary biology. Why cooperate if defection potentially reaps greater benefits? It is commonly assumed that spatial structure (limited dispersal) aligns the interests of mutualistic partners. But does spatial structure consistently promote cooperation? Here, we formally model the role of spatial structure in maintaining mutualism. We show theoretically that spatial structure can actually disfavor cooperation by limiting the suite of potential partners. The effect of spatial structuring depends on the scale (fine or coarse level) at which hosts reward their partners. We then test our predictions by using molecular methods to track the abundance of competing, closely related, cooperative, and less cooperative arbuscular mycorrhizal (AM) fungal symbionts on host roots over multiple generations. We find that when spatial structure is reduced by mixing soil, the relative success of the more cooperative AM fungal species increases. This challenges previous suggestions that high spatial structuring is critical for stabilizing cooperation in the mycorrhizal mutualism. More generally, our results show, both theoretically and empirically, that contrary to expectations, spatial structuring can select against cooperation.
解释物种之间的互利共生合作仍然是进化生物学的一个主要挑战。如果背叛可能带来更大的好处，为什么还要合作呢？通常认为，空间结构（有限的扩散）使互利共生伙伴的利益一致。但是，空间结构是否始终促进合作？在这里，我们从理论上正式研究了空间结构在维持互利共生中的作用。我们表明，空间结构实际上可以通过限制潜在合作伙伴的范围来不利于合作。结构的空间结构取决于宿主奖励其伙伴的尺度（精细或粗糙水平）。然后，我们通过使用分子方法在多个世代中跟踪宿主根系上竞争，密切相关，合作和不太合作的丛枝菌根（AM）真菌共生体的丰度，来检验我们的预测。我们发现，当通过混合土壤减少空间结构时，更合作的 AM 真菌物种的相对成功增加。这挑战了先前的建议，即高空间结构对于稳定共生中合作至关重要。更一般地说，我们的理论和经验结果表明，与预期相反，空间结构可以选择不利于合作。
