Long-term experimental evolution in Escherichia coli. XI. Rejection of non-transitive interactions as cause of declining rate of adaptation
Long-term experimental evolution in Escherichia coli. XI. Rejection of non-transitive interactions as cause of declining rate of adaptation
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DOI：
10.1186/1471-2148-2-19
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发表时间：
2002-10-30
影响因子：
3.4
通讯作者：
Lenski, Richard E.
中科院分区：
文献类型：
--
作者：
de Visser, J. Arjan G. M.;Lenski, Richard E.
背景：大肠杆菌实验种群已经在统一的环境中进化了20,000代。在这段时间里，通过与祖先在那个环境中的竞争来衡量，他们的改良率已经大幅下降。这种减速被解读为细菌在健康环境中接近顶峰或平台期。或者，这种减速可能是由非传递性的竞争相互作用引起的，特别是如果较晚的基因型直接竞争，那么相对于较早的基因型，测量到的优势将更大。结果：为了区分这两个假说，我们使用其中一条进化线路进行了大量的竞争。21个样本以1000代的间隔获得，每个样本与5个以5000代间隔隔离的带有遗传标记的克隆竞争，复制三倍。在传递模型和非传递模型下，将这315个两两比赛的相对适应值模式与预期进行了比较，后者的结构产生了观察到的相对于祖先的适应度减速。总体而言，通过直接竞争衡量的后代人和前代人的相对适合度与分别与祖先竞争得出的适合度很好地吻合。因此，这些数据支持传递性模型。结论：非传递性竞争交互作用不是该种群进化的主要特征。相反，其适应度改善速度的明显减速表明，种群早期合并了大多数提供最大收益的突变，随后依赖于数量较少、效果较小或两者兼而有之的有益突变。
Background: Experimental populations of Escherichia coli have evolved for 20,000 generations in a uniform environment. Their rate of improvement, as measured in competitions with the ancestor in that environment, has declined substantially over this period. This deceleration has been interpreted as the bacteria approaching a peak or plateau in a fitness landscape. Alternatively, this deceleration might be caused by non-transitive competitive interactions, in particular such that the measured advantage of later genotypes relative to earlier ones would be greater if they competed directly.Results: To distinguish these two hypotheses, we performed a large set of competitions using one of the evolved lines. Twenty-one samples obtained at 1,000-generation intervals each competed against five genetically marked clones isolated at 5,000-generation intervals, with three-fold replication. The pattern of relative fitness values for these 315 pairwise competitions was compared with expectations under transitive and non-transitive models, the latter structured to produce the observed deceleration in fitness relative to the ancestor. In general, the relative fitness of later and earlier generations measured by direct competition agrees well with the fitness inferred from separately competing each against the ancestor. These data thus support the transitive model.Conclusion: Non-transitive competitive interactions were not a major feature of evolution in this population. Instead, the pronounced deceleration in its rate of fitness improvement indicates that the population early on incorporated most of those mutations that provided the greatest gains, and subsequently relied on beneficial mutations that were fewer in number, smaller in effect, or both.