Hibbett D S, Gilbert L B, Donoghue M J
Harvard University Herbaria, Cambridge, Massachusetts 02138, USA.
Nature. 2000 Sep 28;407(6803):506-8. doi: 10.1038/35035065.
Mycorrhizae, the symbiotic associations of plant roots and fungal hyphae, are classic examples of mutualisms. In these ecologically important associations, the fungi derive photosynthetic sugars from their plant hosts, which in turn benefit from fungus-mediated uptake of mineral nutrients. Early views on the evolution of symbioses suggested that all long-term, intimate associations tend to evolve toward mutualism. Following this principle, it has been suggested that mycorrhizal symbioses are the stable derivatives of ancestral antagonistic interactions involving plant parasitic fungi. Alternatively, mutualisms have been interpreted as inherently unstable reciprocal parasitisms, which can be disrupted by conflicts of interest among the partners. To determine the number of origins of mycorrhizae, and to assess their evolutionary stability, it is necessary to understand the phylogenetic relationships of the taxa involved. Here we present a broad phylogenetic analysis of mycorrhizal and free-living homobasidiomycetes (mushroom-forming fungi). Our results indicate that mycorrhizal symbionts with diverse plant hosts have evolved repeatedly from saprotrophic precursors, but also that there have been multiple reversals to a free-living condition. These findings suggest that mycorrhizae are unstable, evolutionarily dynamic associations.
菌根是植物根系与真菌菌丝的共生联合体，是互利共生的经典例子。在这些具有重要生态意义的联合体中，真菌从其植物宿主那里获取光合糖，而植物宿主则从真菌介导的矿质养分吸收中受益。早期关于共生关系进化的观点认为，所有长期、紧密的联合体都倾向于朝着互利共生的方向进化。按照这一原则，有人提出菌根共生是涉及植物寄生真菌的祖先拮抗相互作用的稳定衍生物。另外，互利共生也被解释为本质上不稳定的相互寄生关系，可能会因伙伴之间的利益冲突而被破坏。为了确定菌根的起源数量，并评估它们的进化稳定性，有必要了解相关分类群的系统发育关系。在这里，我们对菌根和自由生活的同担子菌（形成蘑菇的真菌）进行了广泛的系统发育分析。我们的结果表明，与不同植物宿主形成菌根的共生体多次从腐生前身进化而来，但也有多次逆转到自由生活状态。这些发现表明菌根是不稳定的、具有进化动态性的联合体。
