Evolution of Archaebacteria: Phylogenetic Relationships Among Archaebacteria, Eubacteria, and Eukaryotes
All extant organisms are thought to be classified into three primary kingdoms (urkingdoms), eubacteria, archaebacteria, and eukaryotes. It is generally impossible to know the correct evolutionary relationships among the three urkingdoms from analysis based on a comparison of a single molecule from a variety of extant species, because the root of molecular phylogenetic tree inferred from such a comparison could not be determined uniquely. We overcame this difficulty by comparing pairs of duplicated genes, elongation factors Tu and G and the catalytic and noncatalytic subunits of ATPase, which are thought to have diverged by gene duplication before divergence of the urkingdoms. Using each protein pair, we inferred a composite phylogenetic tree with two clusters corresponding to different proteins, from which the evolutionary relationship among the urkingdoms is determined uniquely. The inferred composite trees has revealed that all the major groups of archaebacteria (extreme halophiles, extreme thermophiles and methanogens) are more closely related to eukaryotes than to eubacteria. Furthermore, branching patterns of the three major groups of archaebacteria and eukaryotes were examined in detail. It has been shown that the three archaebacterial groups possibly form a single cluster. Phylogenetic positions of three nodes leading to these four groups, however, are likely to be very close to one another.
KeywordsTree Topology Bootstrap Probability Composite Tree Methanosarcina Barkeri Extreme Thermophile
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