Department of Integrative Biology, University of Texas at Austin, Texas 78712; email:
Annu Rev Microbiol. 2014;68:195-215. doi: 10.1146/annurev-micro-091213-112901. Epub 2014 Jun 2.
Starting in 2006, surprisingly tiny genomes have been discovered from numerous bacterial symbionts of insect hosts. Despite their size, each retains some genes that enable provisioning of limiting nutrients or other capabilities required by hosts. Genome sequence analyses show that genome reduction is an ongoing process, resulting in a continuum of sizes, with the smallest genome currently known at 112 kilobases. Genome reduction is typical in host-restricted symbionts and pathogens, but the tiniest genomes are restricted to symbionts required by hosts and restricted to specialized host cells, resulting from long coevolution with hosts. Genes are lost in all functional categories, but core genes for central informational processes, including genes encoding ribosomal proteins, are mostly retained, whereas genes underlying production of cell envelope components are especially depleted. Thus, these entities retain cell-like properties but are heavily dependent on coadaptation of hosts, which continuously evolve to support the symbionts upon which they depend.
从 2006 年开始，人们从昆虫宿主的许多细菌共生体中发现了令人惊讶的小基因组。尽管它们很小，但每个基因组都保留了一些基因，这些基因能够为宿主提供有限的营养物质或其他所需的能力。基因组序列分析表明，基因组的缩小是一个持续的过程，导致了大小的连续体，目前已知最小的基因组大小为 112 千碱基。在宿主限制共生体和病原体中，基因组的缩小是很常见的，但最小的基因组仅限于宿主所需的共生体，并限于专门的宿主细胞，这是与宿主长期共同进化的结果。所有功能类别的基因都丢失了，但中心信息过程的核心基因，包括编码核糖体蛋白的基因，大多保留了下来，而细胞包膜成分产生的基因则特别缺失。因此，这些实体保留了类似细胞的特性，但严重依赖于宿主的共同适应，宿主不断进化以支持它们所依赖的共生体。
