Sloan Daniel B, Nakabachi Atsushi, Richards Stephen, Qu Jiaxin, Murali Shwetha Canchi, Gibbs Richard A, Moran Nancy A
Department of Biology, Colorado State University.
Mol Biol Evol. 2014 Apr;31(4):857-71. doi: 10.1093/molbev/msu004. Epub 2014 Jan 6.
Bacteria confined to intracellular environments experience extensive genome reduction. In extreme cases, insect endosymbionts have evolved genomes that are so gene-poor that they blur the distinction between bacteria and endosymbiotically derived organelles such as mitochondria and plastids. To understand the host's role in this extreme gene loss, we analyzed gene content and expression in the nuclear genome of the psyllid Pachypsylla venusta, a sap-feeding insect that harbors an ancient endosymbiont (Carsonella) with one of the most reduced bacterial genomes ever identified. Carsonella retains many genes required for synthesis of essential amino acids that are scarce in plant sap, but most of these biosynthetic pathways have been disrupted by gene loss. Host genes that are upregulated in psyllid cells housing Carsonella appear to compensate for endosymbiont gene losses, resulting in highly integrated metabolic pathways that mirror those observed in other sap-feeding insects. The host contribution to these pathways is mediated by a combination of native eukaryotic genes and bacterial genes that were horizontally transferred from multiple donor lineages early in the evolution of psyllids, including one gene that appears to have been directly acquired from Carsonella. By comparing the psyllid genome to a recent analysis of mealybugs, we found that a remarkably similar set of functional pathways have been shaped by independent transfers of bacterial genes to the two hosts. These results show that horizontal gene transfer is an important and recurring mechanism driving coevolution between insects and their bacterial endosymbionts and highlight interesting similarities and contrasts with the evolutionary history of mitochondria and plastids.
局限于细胞内环境的细菌经历了广泛的基因组缩减。在极端情况下，昆虫内共生菌进化出的基因组基因极度匮乏，以至于模糊了细菌与内共生衍生细胞器（如线粒体和质体）之间的区别。为了了解宿主在这种极端基因丢失中的作用，我们分析了木虱Pachypsylla venusta核基因组中的基因含量和表达情况。木虱是一种吸食树液的昆虫，它携带着一种古老的内共生菌（卡氏菌），其细菌基因组是已鉴定出的缩减程度最大的基因组之一。卡氏菌保留了许多合成植物树液中稀缺的必需氨基酸所需的基因，但这些生物合成途径中的大多数都因基因丢失而被破坏。在容纳卡氏菌的木虱细胞中上调的宿主基因似乎补偿了内共生菌的基因丢失，从而形成了高度整合的代谢途径，这与在其他吸食树液昆虫中观察到的代谢途径相似。宿主对这些途径的贡献是由天然真核基因和细菌基因共同介导的，这些细菌基因是在木虱进化早期从多个供体谱系水平转移而来的，其中包括一个似乎直接从卡氏菌获得的基因。通过将木虱基因组与最近对粉蚧的分析进行比较，我们发现通过细菌基因向这两种宿主的独立转移，形成了一组非常相似的功能途径。这些结果表明，水平基因转移是驱动昆虫与其细菌内共生菌共同进化的一种重要且反复出现的机制，并突出了与线粒体和质体进化历史有趣的异同。
