Leibniz Institute of Freshwater Ecology and Inland Fisheries, Neuglobsow, Germany.
Berlin Brandenburg Institute of Biodiversity, Berlin, Germany.
Mol Biol Evol. 2021 Mar 9;38(3):1040-1059. doi: 10.1093/molbev/msaa273.
Achromatium is large, hyperpolyploid and the only known heterozygous bacterium. Single cells contain approximately 300 different chromosomes with allelic diversity far exceeding that typically harbored by single bacteria genera. Surveying all publicly available sediment sequence archives, we show that Achromatium is common worldwide, spanning temperature, salinity, pH, and depth ranges normally resulting in bacterial speciation. Although saline and freshwater Achromatium spp. appear phylogenetically separated, the genus Achromatium contains a globally identical, complete functional inventory regardless of habitat. Achromatium spp. cells from differing ecosystems (e.g., from freshwater to saline) are, unexpectedly, equally functionally equipped but differ in gene expression patterns by transcribing only relevant genes. We suggest that environmental adaptation occurs by increasing the copy number of relevant genes across the cell's hundreds of chromosomes, without losing irrelevant ones, thus maintaining the ability to survive in any ecosystem type. The functional versatility of Achromatium and its genomic features reveal alternative genetic and evolutionary mechanisms, expanding our understanding of the role and evolution of polyploidy in bacteria while challenging the bacterial species concept and drivers of bacterial speciation.
无色菌是一种大型、超多倍体且是唯一已知的杂合细菌。单个细胞中大约含有 300 种不同的染色体，等位基因多样性远远超过单个细菌属通常所具有的多样性。通过对所有公开可用的沉积物序列档案进行调查，我们表明无色菌在世界范围内很常见，其分布范围涵盖了温度、盐度、pH 值和深度等通常导致细菌形成物种的范围。尽管盐水和淡水无色菌在系统发育上似乎是分开的，但无论其栖息地如何，无色菌属都包含完全相同的、完整的功能基因库。来自不同生态系统（例如，从淡水到咸水）的无色菌细胞出人意料地具有相同的功能，但通过转录仅相关基因的方式，其基因表达模式存在差异。我们认为，环境适应是通过在细胞的数百条染色体上增加相关基因的拷贝数来实现的，而不会丢失不相关的基因，从而保持在任何生态系统类型中生存的能力。无色菌的功能多样性及其基因组特征揭示了替代的遗传和进化机制，扩展了我们对多倍体在细菌中的作用和进化的理解，同时挑战了细菌物种概念和细菌形成物种的驱动因素。
