PROTEIN-BASED PHYLOGENIES SUPPORT A CHIMERIC ORIGIN FOR THE EUKARYOTIC GENOME

The phylogenetic position of the archaebacteria and the place of eukar yotes in the history of life remain a question of debate. Recent studi es based on some protein-sequence data have obtained unusual phylogeni es for these organisms. We therefore collected the protein sequences t hat were available with representatives from each of the major forms o f life: the gram-negative bacteria, gram-positive bacteria, archaebact eria, and eukaryotes. Monophyletic, unrooted phylogenies based on thes e sequence data show that seven of 24 proteins yield a significant gra m-positive-archaebacteria clade/gram-negative-eukaryotic clade. The ph ylogenies for these seven proteins cannot be explained by the traditio nal three-way split of the eukaryotes, archaebacteria, and eubacteria. Nine of the 24 proteins yield the traditional gram-positive-gram-nega tive clade/archaebacteria-eukaryotic clade. The remaining eight protei ns give phylogenies that cannot be statistically distinguished. These results support the hypothesis of a chimeric origin for the eukaryotic cell nucleus formed from the fusion of an archaebacteria and a gram-n egative bacteria.