Keightley Peter D, Otto Sarah P
Institute of Evolutionary Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK.
Nature. 2006 Sep 7;443(7107):89-92. doi: 10.1038/nature05049.
Sex and recombination are widespread, but explaining these phenomena has been one of the most difficult problems in evolutionary biology. Recombination is advantageous when different individuals in a population carry different advantageous alleles. By bringing together advantageous alleles onto the same chromosome, recombination speeds up the process of adaptation and opposes the fixation of harmful mutations by means of Muller's ratchet. Nevertheless, adaptive substitutions favour sex and recombination only if the rate of adaptive mutation is high, and Muller's ratchet operates only in small or asexual populations. Here, by tracking the fate of modifier alleles that alter the frequency of sex and recombination, we show that background selection against deleterious mutant alleles provides a stochastic advantage to sex and recombination that increases with population size. The advantage arises because, with low levels of recombination, selection at other loci severely reduces the effective population size and genetic variance in fitness at a focal locus (the Hill-Robertson effect), making a population less able to respond to selection and to rid itself of deleterious mutations. Sex and recombination reveal the hidden genetic variance in fitness by combining chromosomes of intermediate fitness to create chromosomes that are relatively free of (or are loaded with) deleterious mutations. This increase in genetic variance within finite populations improves the response to selection and generates a substantial advantage to sex and recombination that is fairly insensitive to the form of epistatic interactions between deleterious alleles. The mechanism supported by our results offers a robust and broadly applicable explanation for the evolutionary advantage of recombination and can explain the spread of costly sex.
有性生殖和重组现象广泛存在，但对这些现象的解释一直是进化生物学中最困难的问题之一。当种群中的不同个体携带不同的有利等位基因时，重组具有优势。通过将有利等位基因组合到同一条染色体上，重组加速了适应过程，并通过穆勒棘轮效应阻止有害突变的固定。然而，只有当适应性突变率很高时，适应性替代才有利于有性生殖和重组，而且穆勒棘轮效应只在小种群或无性繁殖种群中起作用。在这里，通过追踪改变有性生殖和重组频率的修饰基因座等位基因的命运，我们发现针对有害突变等位基因的背景选择为有性生殖和重组提供了一种随种群大小增加的随机优势。这种优势的产生是因为，在重组水平较低时，其他基因座的选择会严重降低有效种群大小和焦点基因座上适合度的遗传方差（希尔 - 罗伯逊效应），使种群对选择的响应能力降低，并难以清除有害突变。有性生殖和重组通过组合中等适合度的染色体来创造相对不含（或携带）有害突变的染色体，从而揭示了隐藏在适合度中的遗传方差。有限种群内遗传方差的这种增加改善了对选择的响应，并为有性生殖和重组带来了显著优势，这种优势对有害等位基因之间上位性相互作用的形式相当不敏感。我们结果所支持的机制为重组的进化优势提供了一个有力且广泛适用的解释，并能解释代价高昂的有性生殖的传播。
