UBC Theses and Dissertations
A genomic survey of two dinotoms Imanian, Behzad
Endosymbiosis has played a major role in shaping eukaryotic cells, their success and diversity. At the base of the eukaryotic tree, an α-proteobacterium endosymbiont in a protoeukaryotic cell was converted into the mitochondrion through its reductive evolution, endosymbiotic gene transfer (EGT) and the development of a protein targeting system to direct the products of the transferred genes to this organelle. Similar events mark the plastid evolution from a cyanobacterium. However, the primary endosymbiosis of plastid, unlike the mitochondrion, was followed by the secondary and tertiary movement of this organelle between eukaryotes through analogous endosymbiotic reduction, EGT and evolution of a protein targeting system and many subsequent independent losses from different eukaryotic lineages. The obligate tertiary diatom endosymbiont in a small group of dinoflagellates called ‘dinotoms’ is exceptional in that it retains most of its ancestral characters including a large nucleus, its own mitochondria, plastids and many other eukaryotic organelles and structures in a large cytoplasm all enclosed in and separated from its dinoflagellate host by a single membrane. This level of conservation of ancestral features in the endosymbiont suggests an early stage of integration. In order to investigate the impacts of endosymbiosis on the organelle genomes and to determine the extent of EGT and the contribution of the host nuclear genome to the proteomes of the organelles, I conducted mass pyrosequencing of the A+T-rich portion of the DNA extracted from two dinotoms, Durinskia baltica and Kryptoperidinium foliaceum, and the SL cDNA library constructed for D. baltica. The plastid and mitochondrial genomes of these two dinotoms were sequenced, and the results indicated that, despite the permanent symbiosis between the host and its endosymbiont in dinotoms and in spite of small variations, the dinotom organelle genomes have changed very little from those of free-living diatoms and dinoflagellates. There was also no sign of EGT to the host in D. baltica, suggesting a strict compartmentalization in which the host mitochondria remain reliant on the host nucleus while the endosymbiont organelles, mitochondria and plastids, stay entirely dependent on the endosymbiont nucleus with no genetic exchange between the host and endosymbiont.
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