Christie Joshua R, Beekman Madeleine
School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW, Australia.
Mol Biol Evol. 2017 Mar 1;34(3):677-691. doi: 10.1093/molbev/msw266.
Eukaryotes carry numerous asexual cytoplasmic genomes (mitochondria and plastids). Lacking recombination, asexual genomes should theoretically suffer from impaired adaptive evolution. Yet, empirical evidence indicates that cytoplasmic genomes experience higher levels of adaptive evolution than predicted by theory. In this study, we use a computational model to show that the unique biology of cytoplasmic genomes-specifically their organization into host cells and their uniparental (maternal) inheritance-enable them to undergo effective adaptive evolution. Uniparental inheritance of cytoplasmic genomes decreases competition between different beneficial substitutions (clonal interference), promoting the accumulation of beneficial substitutions. Uniparental inheritance also facilitates selection against deleterious cytoplasmic substitutions, slowing Muller's ratchet. In addition, uniparental inheritance generally reduces genetic hitchhiking of deleterious substitutions during selective sweeps. Overall, uniparental inheritance promotes adaptive evolution by increasing the level of beneficial substitutions relative to deleterious substitutions. When we assume that cytoplasmic genome inheritance is biparental, decreasing the number of genomes transmitted during gametogenesis (bottleneck) aids adaptive evolution. Nevertheless, adaptive evolution is always more efficient when inheritance is uniparental. Our findings explain empirical observations that cytoplasmic genomes-despite their asexual mode of reproduction-can readily undergo adaptive evolution.
真核生物携带众多无性细胞质基因组（线粒体和质体）。由于缺乏重组，无性基因组理论上应会遭受适应性进化受损的问题。然而，实证证据表明，细胞质基因组经历的适应性进化水平高于理论预测。在本研究中，我们使用一个计算模型来表明，细胞质基因组独特的生物学特性——特别是它们在宿主细胞中的组织方式以及单亲（母系）遗传——使它们能够进行有效的适应性进化。细胞质基因组的单亲遗传减少了不同有益替代之间的竞争（克隆干扰），促进了有益替代的积累。单亲遗传还有助于对有害细胞质替代进行选择，减缓了穆勒棘轮效应。此外，单亲遗传通常会减少选择性清除过程中有害替代的遗传搭便车现象。总体而言，单亲遗传通过增加有益替代相对于有害替代的水平来促进适应性进化。当我们假设细胞质基因组遗传是双亲遗传时，减少配子发生过程中传递的基因组数量（瓶颈效应）有助于适应性进化。然而，当遗传是单亲遗传时，适应性进化总是更有效率。我们的研究结果解释了实证观察结果，即细胞质基因组尽管其无性繁殖方式，但仍能轻易地进行适应性进化。
