MOLECULAR-DATA SUGGEST AN EARLY ACQUISITION OF THE MITOCHONDRION ENDOSYMBIONT

The three deepest branching eucaryotic lineages in small subunit ribos omal RNA phylogenies are the amitochondriate Microspora, Metamonada an d Parabasala. They are followed by either the Euglenozoa (e.g. Euglena and Trypanosoma) or the Percolozoa as the first mitochondria-containi ng eucaryotes. To investigate the hypothesis of an even earlier timing of the mitochondrion endosymbiosis we have amplified a partial cpn-60 coding region from the parabasalid Trichomonas vaginalis and the firs t such sequence from a percolozoan, Naegleria fowleri. Analysis of pre dicted protein sequences reveals a high degree of sequence similarity (greater than or equal to 40%) with a selection of published bacterial and mitochondrial cpn-60s for both taxa. Both sequences were recovere d within a strongly supported monophyletic group, otherwise defined by mitochondrial sequences, which systematically clustered with alpha-pr oteobacteria. These results provide compelling evidence that the ances tor of T. vaginalis once contained the endosymbiont which gave rise to mitochondria, and suggest that this symbiosis probably occurred befor e the Trichomonas lineage diverged from the main eukaryote trunk. It a lso makes feasible the published hypothesis that the Trichomonas hydro genosome might represent a biochemically modified mitochondrion. Analy sis of the N. fowleri cpn-60 did not support the hypothesis that the m itochondrion-containing Percolozoa represent an earlier branch in the cpn-60 tree than Trichomonas or Trypanosoma.