Center for Insect Science, University of Arizona, Tucson, AZ 85721, USA.
Curr Biol. 2011 Aug 23;21(16):1366-72. doi: 10.1016/j.cub.2011.06.051. Epub 2011 Aug 11.
Highly reduced genomes of 144-416 kilobases have been described from nutrient-provisioning bacterial symbionts of several insect lineages [1-5]. Some host insects have formed stable associations with pairs of bacterial symbionts that live in specialized cells and provide them with essential nutrients; genomic data from these systems have revealed remarkable levels of metabolic complementarity between the symbiont pairs [3, 4, 6, 7]. The mealybug Planococcus citri (Hemiptera: Pseudococcidae) contains dual bacterial symbionts existing with an unprecedented organization: an unnamed gammaproteobacteria, for which we propose the name Candidatus Moranella endobia, lives inside the betaproteobacteria Candidatus Tremblaya princeps [8]. Here we describe the complete genomes and metabolic contributions of these unusual nested symbionts. We show that whereas there is little overlap in retained genes involved in nutrient production between symbionts, several essential amino acid pathways in the mealybug assemblage require a patchwork of interspersed gene products from Tremblaya, Moranella, and possibly P. citri. Furthermore, although Tremblaya has the smallest cellular genome yet described, it contains a genomic inversion present in both orientations in individual insects, starkly contrasting with the extreme structural stability typical of highly reduced bacterial genomes [4, 9, 10].
已有研究描述了来自多个昆虫谱系的营养供应细菌共生体的高度简化基因组，大小为 144-416 千碱基[1-5]。一些宿主昆虫与生活在专门细胞中并为其提供必需营养物质的一对细菌共生体形成了稳定的联系；这些系统的基因组数据揭示了共生体对之间显著的代谢互补水平[3,4,6,7]。桔全爪螨（半翅目：粉虱科）含有双重细菌共生体，其存在的组织形式前所未有的：一种未命名的γ变形菌，我们将其命名为 Moranella endobia，生活在β变形菌 Candidatus Tremblaya princeps 内部[8]。在这里，我们描述了这些不寻常嵌套共生体的完整基因组和代谢贡献。我们表明，尽管共生体之间在参与营养生产的保留基因方面几乎没有重叠，但桔全爪螨共生体中存在几种必需氨基酸途径，需要 Tremblaya、Moranella 和可能的 P. citri 中的基因产物相互交错。此外，尽管 Tremblaya 具有迄今为止描述的最小细胞基因组，但它包含一个在个体昆虫中以两种取向存在的基因组倒位，这与高度简化的细菌基因组典型的极端结构稳定性形成鲜明对比[4,9,10]。
