Department of Biology, University of Rochester, Rochester, NY 14623, USA.
J Mol Evol. 2010 Dec;71(5-6):437-43. doi: 10.1007/s00239-010-9397-0. Epub 2010 Oct 12.
In previous work (Betancourt, Genetics 181:1535, 2009), I propagated three large laboratory populations of an RNA phage (MS2) as they adapted to a controlled laboratory environment. These populations were large enough so that evolution might be expected to be mostly repeatable, but they nevertheless fixed different suites of mutations over the course of the experiment. Here, I investigate one possible explanation for these results: epistasis, in which the effect of a mutation depends on its genetic background, may have prevented populations with different initial substitutions from fixing the same set of subsequent mutations. I show that two mutations that previously occurred in different genetic backgrounds are beneficial on either background. This result suggests that sign epistasis-in which a mutation is beneficial on one background, but deleterious on another-is not the cause of different evolutionary trajectories observed in the Betancourt (2009) experiment. However, they can be explained by either magnitude epistasis-in which mutations have stronger or weaker beneficial effects depending on the background-or by the simultaneous fixation of multiple beneficial mutations. Surprisingly, the large populations of the previous experiment showed less parallel evolution than the small populations of this experiment, which lends support to the fixation of multiple beneficial mutations contributing to the patterns seen in both experiments.
在之前的工作（Betancourt，Genetics 181:1535, 2009）中，我繁殖了三个大型实验室种群的 RNA 噬菌体（MS2），让它们适应受控的实验室环境。这些种群足够大，因此可以预期进化主要是可重复的，但在实验过程中，它们确实固定了不同的突变组合。在这里，我研究了这些结果的一个可能解释：上位性，即突变的效应取决于其遗传背景，可能阻止了具有不同初始替代的种群固定相同的后续突变组合。我表明，以前在不同遗传背景中发生的两个突变在任一个背景下都是有益的。这一结果表明，符号上位性——即在一个背景下有益，但在另一个背景下有害——不是导致 Betancourt（2009）实验中观察到的不同进化轨迹的原因。然而，这可以通过幅度上位性来解释，即突变的有益效应取决于背景的强弱，或者通过同时固定多个有益突变来解释。令人惊讶的是，之前实验的大型种群表现出的平行进化程度低于本实验的小型种群，这支持了多个有益突变的固定有助于解释两个实验中观察到的模式。
