Phylogenetic place of archaebacteria and the origin of eukaryotes

通过对重复基因复合树的最大似然分析，证实了古细菌更接近真核生物而不是真细菌。关于古细菌三大类之间的关系，有两种可能；1)古细菌是单系的，2)eocyte更接近真核生物，但其他古细菌如产甲烷菌和盐细菌与这些类群有远亲关系。没有线粒体的原生动物的系统发育定位对于理解真核细胞的早期进化至关重要。以往有关这一问题的研究大多是利用小亚基核糖体RNA （SrRNA）序列完成的。我们已经指出了srrna分析中固有的一个严重问题；也就是说，当G+C压力作用于所研究的任何生物体的基因组时，基于SrRNA的分子系统发育有时会产生误导。我们进一步证明，即使在这种情况下，保守蛋白的氨基酸序列数据，如延伸因子和RNA聚合酶，也不存在这个问题，并且给出了比SrRNA更可靠的树。我们对线粒体原生动物贾第鞭毛虫的延伸因子ef -1 α基因进行了测序。从数据的最大似然分析，我们已经表明，这种生物可能代表了真核生物进化的一个早期分支。

From maximum likelihood analyses of composite trees of duplicated genes, it has ben confirmed that archaebacteria are closer to eukaryotes rather than to eubacteria. Concerning the relationship among the tree major groups of archaebacteria, there remains two possibilities; 1) the archaebacteria are monophyletic, and 2) eocyte is closer to eukaryotes, but other archaebacteria such as methanogens and halobacteria are distantly related to these groups.Phylogenetic placings of protozoa without mitochondria are crucial in understanding the early evolution of eukaryotic cells. Previous studies relevant to this problem have been done mostly by using small-subunit ribosomal RNA (SrRNA) sequences. We have pointed out a serious problem inherent in the analyses of SrRNAs; that is, when G+C pressure is operating to the genome of any of the organisms under study, the molecular phylogeny based on the SrRNA is sometimes misleading. We have further shown that, even in such a situation, amino acid sequence data of conservative proteins, such as, elongation factor and RNA polymerase, are free from the problem, and give more reliable trees than SrRNA.We sequenced elongation factor EF-1alpha gene from an amitochondrial protozoan, Giardia lamblia. From a maximum likelihood analysis of the data, we have shown that this organism may represent an early offshoot in the evolution of eukaryotes.