Institute of Evolutionary Biology and Environmental Studies, University of Zürich, 8057 Zürich, Switzerland.
Proc Biol Sci. 2012 Sep 22;279(1743):3706-15. doi: 10.1098/rspb.2012.0942. Epub 2012 Jul 11.
Bacterial genomes commonly contain 'addiction' gene complexes that code for both a toxin and a corresponding antitoxin. As long as both genes are expressed, cells carrying the complex can remain healthy. However, loss of the complex (including segregational loss in daughter cells) can entail death of the cell. We develop a theoretical model to explore a number of evolutionary puzzles posed by toxin-antitoxin (TA) population biology. We first extend earlier results demonstrating that TA complexes can spread on plasmids, as an adaptation to plasmid competition in spatially structured environments, and highlight the role of kin selection. We then considered the emergence of TA complexes on plasmids from previously unlinked toxin and antitoxin genes. We find that one of these traits must offer at least initially a direct advantage in some but not all environments encountered by the evolving plasmid population. Finally, our study predicts non-transitive 'rock-paper-scissors' dynamics to be a feature of intragenomic conflict mediated by TA complexes. Intragenomic conflict could be sufficient to select deleterious genes on chromosomes and helps to explain the previously perplexing observation that many TA genes are found on bacterial chromosomes.
细菌基因组通常包含“成瘾”基因复合物，这些复合物编码毒素和相应的抗毒素。只要这两个基因都表达，携带复合物的细胞就可以保持健康。然而，复合物的丢失（包括子细胞中的分离丢失）可能会导致细胞死亡。我们开发了一个理论模型来探索由毒素-抗毒素（TA）群体生物学提出的许多进化难题。我们首先扩展了早期的结果，证明 TA 复合物可以在质粒上传播，这是对空间结构环境中质粒竞争的一种适应，并强调了亲缘选择的作用。然后，我们考虑了以前不相关的毒素和抗毒素基因在质粒上出现 TA 复合物的情况。我们发现，这些特征之一必须在进化质粒群体遇到的某些但不是所有环境中提供至少最初的直接优势。最后，我们的研究预测，非传递性的“石头剪刀布”动态将是 TA 复合物介导的基因组内冲突的一个特征。基因组内冲突足以选择染色体上的有害基因，并有助于解释以前令人困惑的观察结果，即许多 TA 基因存在于细菌染色体上。
