Edlind T D, Li J, Visvesvara G S, Vodkin M H, McLaughlin G L, Katiyar S K
Department of Microbiology and Immunology, Medical College of Pennsylvania, Philadelphia 19129, USA.
Mol Phylogenet Evol. 1996 Apr;5(2):359-67. doi: 10.1006/mpev.1996.0031.
It has been proposed that certain extant anaerobic protozoa are descended from organisms that diverged early in eukaryotic evolution prior to the acquisition of mitochondria. Among these are the extracellular parasites Giardia lamblia, Trichomonas vaginalis, and Entamoeba histolytica, and the obligately intracellular microsporidia. Phylogenetic analysis of rRNA sequences from these amitochondrial organisms suggests that G. lamblia, T. vaginalis, and microsporidia are near the base of the eukaryotic tree, while E. histolytica clusters with mitochondria-containing species. However, since eukaryotes likely evolved by symbiotic associations, it is important to analyze other sequences which may have independent origins. Unlike ribosomes, microtubules appear to be unique to eukaryotes. Complete gene sequences for the beta-tubulin subunit of microtubules from T. vaginalis, E. histolytica, and the microsporidian Encephalitozoon hellem have recently been determined. Phylogenetic relationships among these, G. lamblia, and 20 additional beta-tubulins were analyzed by distance matrix and parsimony methods, using alpha- and gamma-tubulin outgroups. All analyses placed the E. histolytica sequence at the base of the beta-tubulin evolutionary tree. Similar results were obtained for E. histolytica alpha-tubulin using a less representative set of sequences. In contrast, the E. hellem sequence branched considerably higher, within the lineage containing animal and fungal beta-tubulins. Possible explanations are considered for these unexpected differences between the beta-tubulin and rRNA trees.
有人提出，某些现存的厌氧原生动物是在真核生物进化早期线粒体获得之前就已分化的生物的后代。其中包括细胞外寄生虫蓝氏贾第鞭毛虫、阴道毛滴虫和溶组织内阿米巴，以及专性细胞内的微孢子虫。对这些无线粒体生物的rRNA序列进行系统发育分析表明，蓝氏贾第鞭毛虫、阴道毛滴虫和微孢子虫靠近真核生物树的基部，而溶组织内阿米巴则与含线粒体的物种聚类。然而，由于真核生物可能是通过共生关系进化而来的，因此分析其他可能有独立起源的序列很重要。与核糖体不同，微管似乎是真核生物特有的。最近已经确定了阴道毛滴虫、溶组织内阿米巴和微孢子虫脑炎微孢子虫微管β-微管蛋白亚基的完整基因序列。使用α-和γ-微管蛋白外群，通过距离矩阵和简约方法分析了这些生物、蓝氏贾第鞭毛虫和另外20种β-微管蛋白之间的系统发育关系。所有分析都将溶组织内阿米巴序列置于β-微管蛋白进化树的基部。使用一组代表性较差的序列对溶组织内阿米巴α-微管蛋白进行分析也得到了类似的结果。相比之下，脑炎微孢子虫序列在包含动物和真菌β-微管蛋白的谱系中分支要高得多。针对β-微管蛋白和rRNA树之间这些意外差异的可能解释进行了探讨。
