School of Botany - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemThe function and evolution of the dinoflagellate mitochondrion(University of Melbourne, 2010)The mitochondrion is a highly evolved, indispensable organelle found in all eukaryotes. This compartment has undergone metabolic and functional changes as cell lineages have diversified and specialised throughout evolution. Alveolata is a well-defined group of single-celled eukaryotes that encompasses related Phyla with extremely diverse lifestyles. Most alveolates belong to one of three main subgroups: predatory ciliates, endoparasitic apicomplexans, and heterotrophic or autotrophic dinoflagellates. Little is known about the biology of the dinoflagellate mitochondrion and studying this compartment offers an opportunity to examine organelle evolution within the alveolate lineage. In this study, I have used the dinoflagellate Karlodinium micrum, as a model to examine mitochondrial evolution. I have investigated the evolution of (1) genes, (2) biochemical functions and (3) protein targeting mechanisms of this organelle. Organelles can replace and gain genes by endosymbiotic gene transfer (EGT, genes derived from an endosymbiont) and lateral gene transfer (LOT, genes derived from an external source). Dinoflagellates have shown a unique propensity to replace their plastids with plastids of other algae during evolution and K. micrum represents a dinoflagellate lineage that has replaced its ancestral plastid with an endosymbiont derived from a haptophyte. In this case, haptophyte endosymbiont plastid genes are located in the dinoflagellate nucleus, providing evidence of EGT in this system. I have assessed if the mitochondrial proteome of K. micrum has been remodelled by EGT and/or LTG. Genes encoding mitochondrial proteins have been identified from a K. micrum expressed sequence tag library and their evolutionary origins inferred by phylogenetics. Several mitochondrial genes are derived from an external source but none originate from the haptophyte endosymbiont, indicating that the K. micrum mitochondrial proteome has been minimally impacted by this endosymbiotic event, but is nevertheless genetically dynamic. Plasmodium falciparum, the disease agent that causes malaria, is a member of Apicomplexa. This protist has a mitochondrion that has been described as metabolically reduced compared to canonical mitochondria, change that has been attributed to the parasitic lifestyle this organism leads. In this thesis, I test whether or not perceived reduction in apicomplexan mitochondrial metabolism is a result of parasitism. A putative metabolic map of the dinoflagellate mitochondrion has been constructed and compared to what is currently known about the mitochondrial biochemistry of closely related apicomplexan parasites and a free living basal alveolate, the ciliate Tetrahymena thermophila. This is the first report of a broad analysis of the mitochondrial metabolism of a dinoflagellate. The mitochondrion of K. micrum shows broad metabolic conservation, having retained pathways implicated in ATP generation by oxidative phosphorylation. Several changes in the metabolism of the P. falciparum mitochondrion were also observed in K. micrum and/or T. thermophila, suggesting that these modifications are not due to parasitism. The presence of most components of the tricarboxylic acid cycle, in addition to what is most likely a functional electron transport chain and ATP synthase complex in both dinoflagellates and P. falciparum indicates that the mitochondrion of the Plasmodium parasite is probably implicated in ATP generation by oxidative phosphorylation. The diversification of dinoflagellates has been accompanied by considerable changes in plastid protein targeting signals, but it is unclear whether or not mitochondrial protein targeting in this lineage has also been modified. In the final experimental chapter of this thesis I have assessed the conservation of mitochondrial protein import mechanisms in dinoflagellates. Genes for K. micrum mitochondrial proteins have been analysed for mitochondrial protein targeting signals using bioinformatic tools, and the function of these signals has been tested using the reporter molecule green fluorescent protein (GFP) and a heterologous yeast expression system. Amino-terminal and internal mitochondrial targeting signals of K. micrum mitochondrial precursors are sufficiently conserved for recognition and import into yeast mitochondria, indicating that dinoflagellate mitochondrial protein targeting signals have been highly conserved, since early in eukaryotic diversification. Overall, my investigations of the dinoflagellate mitochondrion, and this broad comparative analysis of alveolate mitochondria has shown that aspects of mitochondrial biology (ie. mitochondrial gene compliment, mitochondrial biochemistry and function, and mitochondrial protein targeting) have evolved differently during the diversification of alveolates. Alveolate mitochondria are genetically flexible, having experienced gene gains and gene losses. This variation is not always accompanied by functional divergence, and does not necessarily reflect the lifestyle/nutritional requirements of the host. Thus while the mitochondrion is clearly an innovative compartment, its evolutionary behaviour cannot be characterised based on any one aspect of its cell biology alone.