Embley T M, Finlay B J
Microbiology Group, Natural History Museum, London, UK.
Antonie Van Leeuwenhoek. 1993;64(3-4):261-71. doi: 10.1007/BF00873086.
The identities and taxonomic diversity of the endosymbiotic methanogens from the anaerobic protozoa Metopus contortus, Metopus striatus, Metopus palaeformis, Trimyema sp. and Pelomyxa palustris were determined by comparative analysis of their 16S ribosomal RNA sequences. Fluorescent oligonucleotide probes were designed to bind to the symbiont rRNA sequences and to provide direct visual evidence of their origins from methanogenic archaea contained within the host cells. Confocal microscopy was used to analyze the morphology of the endosymbionts in whole cells of Metopus palaeformis, Metopus contortus, Trimyema sp, and Cyclidium porcatum. The endosymbionts are taxonomically diverse and are drawn from three different genera; Methanobacterium, Methanocorpusculum and Methanoplanus. In every case the symbionts are closely related to, but different from, free-living methanogens for which sequences are available. It is thus apparent that symbioses have been formed repeatedly and independently. Ciliates which are unrelated to each other (Trimyema sp. and Metopus contortus) may contain symbionts which are closely related, and congeneric ciliates (Metopus palaeformis and M. contortus) may contain symbionts which are distantly related to each other. This suggests that some of the symbiotic associations must be relatively recent. For example, at least one of the symbioses in Metopus must postdate the speciation of M. palaeformis and M. contortus. Despite this, Metopus contortus, Trimyema sp., Cyclidium porcatum and their respective endosymbionts show sophisticated morphological interactions which probably facilitate the exchange of materials between the partners.
通过对来自厌氧原生动物扭形扁虫（Metopus contortus）、条纹扁虫（Metopus striatus）、古形扁虫（Metopus palaeformis）、三毛滴虫（Trimyema sp.）和沼泽多核变形虫（Pelomyxa palustris）的内共生产甲烷菌的16S核糖体RNA序列进行比较分析，确定了它们的身份和分类多样性。设计了荧光寡核苷酸探针，使其与共生体的rRNA序列结合，并为其源自宿主细胞内产甲烷古菌提供直接的视觉证据。利用共聚焦显微镜分析了古形扁虫、扭形扁虫、三毛滴虫和猪小环虫（Cyclidium porcatum）全细胞内共生体的形态。这些内共生体在分类学上具有多样性，分属于三个不同的属：甲烷杆菌属（Methanobacterium）、甲烷微粒菌属（Methanocorpusculum）和甲烷平面菌属（Methanoplanus）。在每种情况下，共生体都与现有序列的自由生活产甲烷菌密切相关，但又有所不同。因此很明显，共生关系是反复独立形成的。彼此无关的纤毛虫（三毛滴虫和扭形扁虫）可能含有密切相关的共生体，而同属的纤毛虫（古形扁虫和扭形扁虫）可能含有彼此关系较远的共生体。这表明一些共生关系肯定是相对较新形成的。例如，扁虫属中的至少一种共生关系肯定发生在古形扁虫和扭形扁虫物种形成之后。尽管如此，扭形扁虫、三毛滴虫、猪小环虫及其各自的内共生体显示出复杂的形态相互作用，这可能有助于共生伙伴之间的物质交换。
