We constructed genomic trees based on the presence and absence of families of protein-encoding genes observed in 55 prokaryotic and five eukaryotic genomes. There are features of the genomic trees that are not congruent with typical rRNA phylogenetic trees. In the bacteria, for example, Deinococcus radiodurans associates with the Gram-positive bacteria, a result that is also seen in some other phylogenetic studies using whole genome data. In the Archaea, the methanogens plus Archaeoglobus form a united clade and the Euryarchaeota are divided with the two Thermoplasma genomes and Halobacterium sp. falling below the Crenarchaeota. While the former appears to be an accurate representation of methanogen-relatedness, the misplacement of Halobacterium may be an artefact of parsimony. These results imply the last common ancestor of the Archaea was not a methanogen, leaving sulphur reduction as the most geochemically plausible metabolism for the base of the archaeal crown group. It also suggests that methanogens were not a component of the Earth's earliest biosphere and that their origin occurred sometime during the Archean. In the Eukarya, the parsimony analysis of five Eukaryotes using the Crenarchaeota as an outgroup seems to counter the Ecdysozoa hypothesis, placing Caenorhabditis elegans (Nematoda) below the common ancestor of Drosophila melanogaster (Arthropoda) and Homo sapiens (Chordata) even when efforts are made to counter the possible effects of a faster rate of sequence evolution for the C. elegans genome. Further analysis, however, suggests that the gene loss of ‘animal’ genes is highest in C. elegans and is obscuring the relationships of these organisms.
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