Studies on the phylogeny and gene structure of early-branching eukaryotes

The Archezoa hypothesis holds that several living protist groups constitute primitive eukaryotic lineages that diverged from the main eukaryotic lineage prior to the endosymbiotic origin of mitochondria. Several aspects of this hypothesis were tested.Firstly, elongation factor 1To evaluate the "introns-late" claim that spliceosomal introns are a derived feature of eukaryotic genome absent in the earliest protist lineages, triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase genes were obtained from several of the putatively early-branching eukaryotic groups. These, along with other homologs, were assembled into intron datasets and a novel maximum likelihood method was used to evaluate the likelihood of "introns-early" and "introns-late" models. The latter view was shown to confer the greatest probability on the data.In the final two studies, the Archezoa hypothesis was directly tested. A mitochondrial-like chaperonin 60 gene was cloned and sequenced from the early-branching amitochondriate protist Trichomonas vaginalis. Phylogenetic analyses of datasets including this sequence coupled with the hydrogenosomal location for the protein argues strongly for a common ancestry for hydrogenosomes and mitochondria. In the last study, several of the early-branching groups were shown to possess iron superoxide dismutase genes that cluster with proteobacterial homologs. The means by which these genes were acquired by these protists are unclear, but the possibility exists that some or all of them may derive from the mitochondrial endosymbiosis.