Studies on the phylogeny and gene structure of early-branching eukaryotes.
Date
1997
Authors
Roger, Andrew James.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
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 1$\alpha$ and $\beta$-tubulin genes were developed as phylogenetic markers for early eukaryote evolution by amplifying several homologs from mitochondriate and amitochondriate protists. The phylogenies of these genes suggest that, after a few early branchings, a deep split occurred in eukaryote evolution that resulted in the emergence of two distinct protist/multicell superclusters. Interestingly, the amitochondriate Microsporidia are strongly placed as a sister group to the Fungi in the $\beta$-tubulin tree. Statistical tests using both datasets, the shared presence of an insertion in microsporidian, fungal and metazoan EF-1$\alpha$ genes as well as ultrastructural considerations suggest that this phylogenetic position is the best supported by the data.
To 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.
Thesis (Ph.D.)--Dalhousie University (Canada), 1997.
Firstly, elongation factor 1$\alpha$ and $\beta$-tubulin genes were developed as phylogenetic markers for early eukaryote evolution by amplifying several homologs from mitochondriate and amitochondriate protists. The phylogenies of these genes suggest that, after a few early branchings, a deep split occurred in eukaryote evolution that resulted in the emergence of two distinct protist/multicell superclusters. Interestingly, the amitochondriate Microsporidia are strongly placed as a sister group to the Fungi in the $\beta$-tubulin tree. Statistical tests using both datasets, the shared presence of an insertion in microsporidian, fungal and metazoan EF-1$\alpha$ genes as well as ultrastructural considerations suggest that this phylogenetic position is the best supported by the data.
To 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.
Thesis (Ph.D.)--Dalhousie University (Canada), 1997.
Keywords
Biology, Molecular., Biology, Genetics., Biology, Microbiology., Biology, Zoology.