Program in Ecology, Evolutionary Biology & Behavior, Michigan State University, East Lansing, Michigan 48824, USA.
Evolution. 2010 Oct;64(10):3024-34. doi: 10.1111/j.1558-5646.2010.01049.x. Epub 2010 Aug 19.
The dynamics of host susceptibility to parasites are often influenced by trade-offs between the costs and benefits of resistance. We assayed changes in the resistance to three viruses in six lines of Escherichia coli that had been evolving for almost 45,000 generations in their absence. The common ancestor of these lines was completely resistant to T6, partially resistant to T6* (a mutant of T6 with altered host range), and sensitive to λ. None of the populations changed with respect to resistance to T6, whereas all six evolved increased susceptibility to T6*, probably ameliorating a cost of resistance. More surprisingly, however, the majority of lines evolved complete resistance to λ, despite not encountering that virus during this period. By coupling our results with previous work, we infer that resistance to λ evolved as a pleiotropic effect of a beneficial mutation that downregulated an unused metabolic pathway. The strong parallelism between the lines implies that selection had almost deterministic effects on the evolution of these patterns of host resistance. The opposite outcomes for resistance to T6* and λ demonstrate that the evolution of host resistance under relaxed selection cannot be fully predicted by simple trade-off models.
宿主对寄生虫易感性的变化通常受到抵抗成本和收益之间权衡的影响。我们检测了在不存在的情况下，经过近 45000 代进化的 6 条大肠杆菌系对 3 种病毒的抗性变化。这些系的共同祖先对 T6 完全具有抗性，对 T6*（T6 的一种具有改变宿主范围的突变体）具有部分抗性，对 λ 敏感。所有种群在 T6 抗性方面都没有变化，而所有 6 个种群都对 T6的敏感性增加，可能减轻了抗性的成本。然而，更令人惊讶的是，尽管在这段时间内没有遇到 λ 病毒，但大多数系却完全对 λ 产生了抗性。通过将我们的结果与之前的工作结合起来，我们推断 λ 的抗性是有益突变的多效性效应，该突变下调了未使用的代谢途径。这些宿主抗性模式的演化具有很强的相似性，这意味着选择对这些模式的演化几乎具有决定性的影响。对 T6和 λ 的抗性产生相反的结果表明，在选择放松的情况下，宿主抗性的演化不能完全用简单的权衡模型来预测。
