Du Y, Maslov D A, Chang K P
Department of Microbiology and Immunology, Finch University of Health Sciences/Chicago Medical School, IL 60064.
Proc Natl Acad Sci U S A. 1994 Aug 30;91(18):8437-41. doi: 10.1073/pnas.91.18.8437.
Some trypanosomatid protozoa (order Kinetoplastida) are well known to harbor bacterial endosymbionts. Their phylogenetic positions and evolutionary relationships with the hosts were deduced by comparing the rRNA gene sequences. Earlier, we observed that these symbionts from three Crithidia spp. are identical and are closely related to Bordetella bronchiseptica. We have now sequenced the genes of another endosymbiont and the host protozoan Blastocrithidia culicis. The 16S rRNA genes of the Blastocrithidia and Crithidia symbionts share approximately 97% identity and form a distinct group, branching off the B. bronchiseptica lineage in the beta-division of Proteobacteria. Comparison of their secondary structures in the stem regions suggests compensatory mutations of the symbiont sequences, contributing to their biased base transitions from G to A and C to T. Two putative genes encoding tRNA(Ile) and tRNA(Ala) are highly conserved in the otherwise variable internal transcribed spacer region. Comparisons of the host rRNA gene sequences suggest that the symbiont-containing Crithidia and Blastocrithidia are more akin to each other than to other trypanosomatids. The evidence suggests that Blastocrithidia and Crithidia symbionts descend from a common ancestor, which had presumably entered an ancestral host and thence coevolved with it into different species. We therefore propose naming the symbionts Kinetoplastibacterium blastocrithidii and Kinetoplastibacterium crithidii.
一些锥虫类原生动物（动质体目）因含有细菌内共生体而广为人知。通过比较rRNA基因序列推断出它们与宿主的系统发育位置和进化关系。早些时候，我们观察到来自三种克氏锥虫属物种的这些共生体是相同的，并且与支气管败血博德特氏菌密切相关。我们现在已经对另一种内共生体和宿主原生动物库蚊脆双核虫的基因进行了测序。库蚊脆双核虫和克氏锥虫共生体的16S rRNA基因具有约97%的同一性，并形成一个独特的群体，在变形菌门的β-亚纲中从支气管败血博德特氏菌谱系分支出来。对它们茎区二级结构的比较表明共生体序列存在补偿性突变，这导致了它们从G到A以及从C到T的偏向性碱基转换。在其他可变的内部转录间隔区中，两个假定的编码异亮氨酸tRNA和丙氨酸tRNA的基因高度保守。对宿主rRNA基因序列的比较表明，含有共生体的克氏锥虫和库蚊脆双核虫彼此之间的亲缘关系比与其他锥虫类原生动物更密切。证据表明，库蚊脆双核虫和克氏锥虫的共生体起源于一个共同祖先，这个祖先可能进入了一个祖先宿主，然后与其共同进化成不同的物种。因此，我们建议将这些共生体命名为库蚊脆双核杆菌和克氏锥虫杆菌。
