Department of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195, USA.
Proc Natl Acad Sci U S A. 2010 Feb 2;107(5):2124-9. doi: 10.1073/pnas.0908456107. Epub 2010 Jan 19.
Mutualistic interactions are taxonomically and functionally diverse. Despite their ubiquity, however, the basic ecological and evolutionary processes underlying their origin and maintenance are poorly understood. A major reason for this is the lack of an experimentally tractable model system. We examine the evolution of an experimentally imposed obligate mutualism between sulfate-reducing and methanogenic microorganisms that have no known history of previous interaction. Twenty-four independent pairings (cocultures) of the bacterium Desulfovibrio vulgaris and the archaeon Methanococcus maripaludis were established and followed for 300 community doublings in two environments, one allowing for the development of a heterogeneous distribution of resources and the other not. Evolved cocultures grew up to 80% faster and were up to 30% more productive (biomass yield per mole of substrate) than the ancestors. The evolutionary process was marked by periods of significant instability leading to extinction of two of the cocultures, but it resulted in more stable, efficient, and productive mutualisms for most replicated pairings. Comparisons of evolved cocultures with those assembled from one evolved mutualist and one ancestral mutualist showed that evolution of both species contributed to improved productivity. Surprisingly, however, overall improvements in growth rate and yield were less than the sum of the individual contributions, suggesting antagonistic interactions between mutations from the coevolved populations. Physical constraints on the transfer of metabolites in the evolution environment affected the evolution of M. maripaludis, but not of D. vulgaris. Together, these results demonstrate that challenges can imperil nascent obligate mutualisms and demonstrate the evolutionary responses that enable their persistence and future evolution.
互利共生相互作用在分类学和功能上都是多种多样的。然而，尽管它们无处不在，但对于它们起源和维持的基本生态和进化过程却知之甚少。造成这种情况的一个主要原因是缺乏可实验处理的模型系统。我们研究了在硫酸盐还原菌和产甲烷菌之间强制形成的实验性专性互利共生的进化，而这两种微生物以前没有已知的相互作用历史。我们建立了 24 个独立的脱硫弧菌和甲烷八叠球菌的配对（共培养物），并在两种环境中进行了 300 次群落倍增的跟踪研究，一种环境允许资源的异质分布发展，另一种环境则不允许。进化后的共培养物的生长速度比祖先快了 80%，生物量产量比祖先高了 30%（每摩尔底物的产量）。进化过程中存在着显著的不稳定性时期，导致其中两个共培养物灭绝，但对于大多数复制的配对，它导致了更稳定、高效和富有成效的互利共生关系。与从一个进化的互利共生体和一个祖先的互利共生体组装而成的进化后的共培养物进行比较表明，两个物种的进化都有助于提高生产力。然而，令人惊讶的是，生长速度和产量的整体提高小于单个贡献的总和，这表明来自共同进化种群的突变之间存在拮抗相互作用。进化环境中代谢物转移的物理限制影响了产甲烷菌的进化，但不影响脱硫弧菌的进化。总的来说，这些结果表明，挑战可能会危及新生的专性互利共生关系，并展示了使它们得以持续存在和未来进化的进化反应。
