Aneuploidy is common in eukaryotes, often leading to decreased fitness. However, evidence from fungi and human tumur cells suggests that specific aneuploidies can be beneficial under stressful conditions and facilitate adaptation. In a previous evolutionary experiment with yeast, populations evolving under heat stress became aneuploid, only to later revert to euploidy after beneficial mutations accumulated. It was therefore suggested that aneuploidy is a “stepping stone” on the path to adaptation. Here, we test this hypothesis. We use Bayesian inference to fit an evolutionary model with both aneuploidy and mutation to the experimental results. We then predict the genotype frequency dynamics during the experiment, demonstrating that most of the evolved euploid population likely did not descend from aneuploid cells, but rather from the euploid wild-type population. Our model shows how the beneficial mutation supply—the product of population size and beneficial mutation rate—determines the evolutionary dynamics: with low supply, much of the evolved population descends from aneuploid cells; but with high supply, beneficial mutations are generated fast enough to outcompete aneuploidy due to its inherent fitness cost. Our results suggest that despite its potential fitness benefits under stress, aneuploidy can be an evolutionary “diversion” rather than a “stepping stone”: it can delay, rather than facilitate, the adaptation of the population, and cells that become aneuploid may leave less descendants compared to cells that remain diploid.

中文翻译：

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非整倍体可能是适应道路上的进化转移

非整倍体在真核生物中很常见，通常会导致适应性下降。然而，来自真菌和人类肿瘤细胞的证据表明，特定的非整倍体在压力条件下可能是有益的，并有助于适应。在之前的酵母进化实验中，在热应激下进化的群体变成了非整倍体，只有在有益突变积累后才恢复到整倍体。因此，有人认为非整倍体是适应之路上的“垫脚石”。在这里，我们测试这个假设。我们使用贝叶斯推理将具有非整倍性和突变的进化模型与实验结果进行拟合。然后，我们预测实验期间的基因型频率动态，证明大多数进化的整倍体群体可能不是来自非整倍体细胞，而是来自整倍体野生型群体。我们的模型显示了有益突变供应（种群规模和有益突变率的乘积）如何决定进化动态：供应量低时，大部分进化种群都来自非整倍体细胞；供应量低时，大部分进化种群都来自非整倍体细胞；但在供应充足的情况下，有益突变的产生速度足以超越非整倍体，因为其固有的适应成本。我们的研究结果表明，尽管非整倍体在压力下具有潜在的健康益处，但它可能是一种进化“转移”而不是“垫脚石”：它可以延迟而不是促进群体的适应，并且变成非整倍体的细胞可能会留下更少的适应能力。后代与保持二倍体的细胞相比。