Data from: The cost of copy number in a selfish genetic element: the 2µM plasmid of Saccharomyces cerevisiae

Many autonomously replicating genetic elements exist as multiple copies within the cell. The copy number of these elements is often assumed to have important fitness consequences for both element and host, yet the forces shaping its evolution are not well understood. The 2µm is a multi-copy plasmid of Saccharomyces yeasts, encoding just four genes that are solely involved in plasmid replication. One simple model for the fitness relationship between yeasts and 2µm is that plasmid copy number evolves as a tradeoff between selection for increased vertical transmission, favoring high copy number, and selection for decreased virulence, favoring low copy number. To test this model, we experimentally manipulated the copy number of the plasmid and directly measured the fitness cost, in terms of growth rate reduction, associated with high plasmid copy number. We find that the fitness burden imposed by the 2µm increases with plasmid copy number, such that each copy imposes a fitness burden of 0.17% (±0.008%), greatly exceeding the cost expected for it to be stably maintained in yeast populations. Our results demonstrate the crucial importance of copy number in the evolution of yeast/2µm associations, and pave the way for future studies examining how selection can shape the cost of multi-copy elements.诸多自主复制的遗传因子在细胞内以多拷贝形式存在。这类遗传因子的拷贝数通常被认为对因子自身与宿主均具有重要的适合度（fitness）影响，然而塑造其进化的选择压力机制仍未被充分阐明。2μm质粒是酵母属（Saccharomyces）的多拷贝质粒，仅编码四个专一参与质粒复制的基因。关于酵母与2μm质粒之间的适合度关联，有一个简单模型认为：质粒拷贝数的进化是两种选择压力权衡的结果——一是选择提升质粒的垂直传递效率，该选择倾向于维持高拷贝数；二是选择降低质粒对宿主的毒性，该选择倾向于维持低拷贝数。为验证该模型，我们通过实验手段调控了该质粒的拷贝数，并直接测定了高质粒拷贝数所带来的适合度代价（以生长速率降低为衡量指标）。本研究发现，2μm质粒带来的适合度负担随质粒拷贝数升高而增加，单份拷贝带来的适合度负担为0.17%（±0.008%），远高于其在酵母种群中稳定维持所需的预期代价。本研究结果证实了拷贝数在酵母与2μm质粒的共生关联进化中的关键作用，并为未来探究选择压力如何塑造多拷贝遗传因子的适合度代价的相关研究铺平了道路。