The evolution of sex is favoured during adaptation to new environments.
The evolution of sex is favoured during adaptation to new environments.
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DOI：
10.1371/journal.pbio.1001317
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发表时间：
2012
期刊：
影响因子：
9.8
通讯作者：
Agrawal AF
中科院分区：
文献类型：
--
作者：
Becks L;Agrawal AF
在对兼性有性轮虫的实验中，适应新环境的种群进化出更高的有性率，因为有性混合会迅速组装适应良好的基因类型。理论和实验都已经证明，性行为可以促进适应，潜在地为性行为带来群体层面的优势。然而，目前尚不清楚这一过程是否有助于解决更困难的问题，即在人群中维持性行为。利用兼性轮虫花瓣臂尾轮虫的实验种群，我们发现，在适应过程中，性化率进化到更高的水平，但随着适应的平台期，性化率下降。为了评估基因混合的适合度结果，我们直接比较了自然出现在我们的实验种群中的有性和无性衍生基因的适合度。当适应压力很大时，有性来源的基因比无性来源的基因更适合，但随着适应速度的放缓，这种模式发生了逆转，与性别比率的进化变化模式相匹配。这些适合度分析测试了性行为的净影响，但不能用来理清是否是因为高性别性血统与不同的等位基因组合或等位基因频率与性行为低性性血统的不同相关(即，分别是“短期”或“长期”影响)，才会产生性选择。我们通过执行额外的操作来从无偏见的双亲(而不是从自然发生的从而进化上有偏见的双亲)获得有性和无性后代阵列的适应度分布，从而推断出这些机制中的哪一种提供了有性优势。我们发现，有证据表明，性破坏了适应性基因组合，导致有性后代的平均适合度较低(即，有性在短期内处于不利地位)。正如理论预测的那样，有性繁殖的优势之所以出现，是因为有性繁殖的后代在适应能力上更加多变，从而能够更快地适应环境。这种“长期优势”跨越了几代人，最终导致了更高的性型适合度。一个多世纪以来，生物学家一直在想，为什么性行为是一种如此常见的生殖模式，因为每个父母的后代数量减少会立即带来2倍的健康成本。最经典的解释是，性爱之所以受到青睐，是因为它有助于产生适应所需的变化。虽然理论模型和间接证据支持这一观点，但没有直接的实验数据，而且远不明显的是，任何这样的优势是否能够平衡性行为的可观成本。利用一种兼性繁殖的轮虫实验种群，我们证明了随着种群适应新的环境，性别比率会在进化中增加。我们表明，性行为创造了一系列不同的基因类型，包括许多非常不适合的，但也有一些非常适合新环境的。尽管这些性衍生后代的平均适合度低于无性恋者，但那些通过性行为产生的适应良好的基因类型对后代的贡献不成比例，导致性行为的遗传倾向最终增加。
In experiments with a facultatively sexual rotifer, populations adapting to novel environments evolve higher rates of sex because sexual mixing quickly assembles well-adapted genotypes. Both theory and experiments have demonstrated that sex can facilitate adaptation, potentially yielding a group-level advantage to sex. However, it is unclear whether this process can help solve the more difficult problem of the maintenance of sex within populations. Using experimental populations of the facultatively sexual rotifer Brachionus calyciflorus, we show that rates of sex evolve to higher levels during adaptation but then decline as fitness plateaus. To assess the fitness consequences of genetic mixing, we directly compare the fitnesses of sexually and asexually derived genotypes that naturally occur in our experimental populations. Sexually derived genotypes are more fit than asexually derived genotypes when adaptive pressures are strong, but this pattern reverses as the pace of adaptation slows, matching the pattern of evolutionary change in the rate of sex. These fitness assays test the net effect of sex but cannot be used to disentangle whether selection on sex arises because highly sexual lineages become associated with different allele combinations or with different allele frequencies than less sexual lineages (i.e., “short-” or “long-term” effects, respectively). We infer which of these mechanisms provides an advantage to sex by performing additional manipulations to obtain fitness distributions of sexual and asexual progeny arrays from unbiased parents (rather than from naturally occurring, and thereby evolutionarily biased, parents). We find evidence that sex breaks down adaptive gene combinations, resulting in lower average fitness of sexual progeny (i.e., a short-term disadvantage to sex). As predicted by theory, the advantage to sex arises because sexually derived progeny are more variable in fitness, allowing for faster adaptation. This “long-term advantage” builds over multiple generations, eventually resulting in higher fitness of sexual types. For well over a century, biologists have wondered why sex is such a common mode of reproduction, given the immediate 2-fold fitness cost entailed by the reduced number of offspring per parent. The most classic explanation is that sex is favoured because it helps to generate the variation necessary for adaptation. While theoretical models and indirect lines of evidence support this idea, there are no direct experimental data and it is far from obvious whether any such advantage could balance the considerable costs of sex. Using experimental populations of a facultatively sexual species of rotifer, we demonstrate that rates of sex evolutionarily increase as populations adapt to novel environments. We show that sex creates a diverse array of genotypes, including many that are quite unfit but also others that are very fit in the new environment. Though the average fitness of these sexually derived offspring is lower than that of asexuals, those well-adapted genotypes generated by sex contribute disproportionately to future generations, causing the genetic propensity for sex to ultimately increase.
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