Data from: Contrasting dynamics of a mutator allele in asexual populations of differing size

Mutators have been shown to hitchhike in asexual populations when the anticipated beneficial mutation supply rate of the mutator subpopulation, NU_b (for subpopulation of size N and beneficial mutation rate U_b) exceeds that of the wild-type subpopulation. Here, we examine the effect of total population size on mutator dynamics in asexual experimental populations of Saccharomyces cerevisiae. Although mutators quickly hitchhike to fixation in smaller populations, mutator fixation requires more and more time as population size increases; this observed delay in mutator hitchhiking is consistent with the expected effect of clonal interference. Interestingly, despite their higher beneficial mutation supply rate, mutators are supplanted by the wild type in very large populations. We postulate that this striking reversal in mutator dynamics is caused by an interaction between clonal interference, the fitness cost of the mutator allele, and infrequent large-effect beneficial mutations in our experimental populations. Our work thus identifies a potential set of circumstances under which mutator hitchhiking can be inhibited in natural asexual populations, despite recent theoretical predictions that such populations should have a net tendency to evolve ever-higher genomic mutation rates.已有研究证实，当增变突变株（mutator）亚种群的预期有益突变供应率NU_b（其中N为亚种群规模，U_b为有益突变率）高于野生型亚种群时，增变突变株可在无性种群中发生遗传搭便车（hitchhike）效应。本研究以酿酒酵母（Saccharomyces cerevisiae）的无性实验种群为对象，探究了总种群规模对增变突变株动态演化的影响。尽管在较小种群中，增变突变株可快速通过搭便车实现种群固定，但随着种群规模扩大，其达到固定所需的时间显著延长；这一观测到的搭便车延迟现象与克隆干扰（clonal interference）的预期效应相符。有趣的是，尽管增变突变株的有益突变供应率更高，但在超大种群中，它们反而会被野生型种群取代。我们推测，增变突变株动态的这一显著反转，是由克隆干扰、增变等位基因的适合度成本，以及本实验种群中罕见的大效应有益突变三者间的相互作用所导致的。综上，尽管近期理论预测无性自然种群整体上倾向于演化出更高的基因组突变率，但本研究明确了一类可抑制增变突变株搭便车现象的潜在环境与种群条件。