1] Laboratory of Microbiology, Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Japan [2] Subsurface Geobiology Advanced Research (SUGAR) Project, Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
Subsurface Geobiology Advanced Research (SUGAR) Project, Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
ISME J. 2014 Jan;8(1):40-51. doi: 10.1038/ismej.2013.131. Epub 2013 Aug 8.
Deep-sea vents harbor dense populations of various animals that have their specific symbiotic bacteria. Scaly-foot gastropods, which are snails with mineralized scales covering the sides of its foot, have a gammaproteobacterial endosymbiont in their enlarged esophageal glands and diverse epibionts on the surface of their scales. In this study, we report the complete genome sequencing of gammaproteobacterial endosymbiont. The endosymbiont genome displays features consistent with ongoing genome reduction such as large proportions of pseudogenes and insertion elements. The genome encodes functions commonly found in deep-sea vent chemoautotrophs such as sulfur oxidation and carbon fixation. Stable carbon isotope ((13)C)-labeling experiments confirmed the endosymbiont chemoautotrophy. The genome also includes an intact hydrogenase gene cluster that potentially has been horizontally transferred from phylogenetically distant bacteria. Notable findings include the presence and transcription of genes for flagellar assembly, through which proteins are potentially exported from bacterium to the host. Symbionts of snail individuals exhibited extreme genetic homogeneity, showing only two synonymous changes in 19 different genes (13 810 positions in total) determined for 32 individual gastropods collected from a single colony at one time. The extremely low genetic individuality in endosymbionts probably reflects that the stringent symbiont selection by host prevents the random genetic drift in the small population of horizontally transmitted symbiont. This study is the first complete genome analysis of gastropod endosymbiont and offers an opportunity to study genome evolution in a recently evolved endosymbiont.
深海热液喷口栖息着各种动物的密集种群，这些动物都有其特定的共生细菌。有鳞腹足类动物是一种带有矿化鳞片覆盖其足部侧面的蜗牛，其扩大的食管腺中有γ变形菌内共生体，其鳞片表面有多种外共生体。在这项研究中，我们报告了γ变形菌内共生体的完整基因组测序结果。该内共生体基因组显示出与正在进行的基因组缩减相关的特征，例如大量的假基因和插入元件。该基因组编码了在深海热液喷口化能自养生物中常见的功能，如硫氧化和碳固定。稳定碳同位素（^13C）标记实验证实了内共生体的化能自养性。该基因组还包含一个完整的氢化酶基因簇，该基因簇可能是从系统发育上较远的细菌中水平转移而来的。值得注意的发现包括鞭毛组装相关基因的存在和转录，通过这些基因，蛋白质可能从细菌输出到宿主。来自单个殖民地的 32 只蜗牛个体的共生体表现出极高的遗传同质性，在总共 19 个不同基因（总计 13810 个位置）中仅发现了两个同义突变。内共生体极低的遗传个体性可能反映了宿主对内共生体的严格选择，防止了水平传播共生体在小种群中的随机遗传漂变。本研究是对腹足类内共生体的首次完整基因组分析，为研究最近进化的内共生体的基因组进化提供了机会。
