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2003 Annual Science Report

Marine Biological Laboratory Reporting  |  JUL 2002 – JUN 2003

Eukaryotic Origins and the Evolution of Cellular Complexity - Eukaryotic rRNA Evolution: Early Diverging Eukaryotes

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A common feature of early diverging eukaryotic lineages is the absence of mitochondria and their ability to thrive in anoxic environments. We have characterized ribosomal RNAs (rRNAs) from several amitochondriate species, including Carpediemonas, which contains double membrane-bounded structures physically resembling hydrogenosomes. In some protists, hydrogenosomes appear to be derived from mitochondria, and molecular analyses of heat shock proteins argue that mitochondria were once present in all protists, including those that live in anoxic environments. Whether or not the first eukaryotes had mitochondria is a key issue that will require extensive molecular analyses of several amitochondriate protists. We already know that Entamoebae and microsporidial genomes contain several coding regions that define proteins that likely came from the ancestral symbiont of mitochondria. We have recently characterized two new amitochondriate protists isolated from anoxic environments. Analyses of ribosomal RNAs from Monopylocystis visvesvarai and Sawyeria marylandensis demonstrate their affinity with heterolobosean amoebae. Both species lack mitochondria but they have organelles provisionally interpreted as hydrogenosomes. Neither tolerate aerobic conditions; however, they do tolerate very low levels of oxygen. These heteroloboseans have unusual nuclear morphologies, and Monopylocystis visvesvarai is the first example of an amitochondriate heterolobosean protist for which a cyst is known. Future studies will focus upon questions that explore the possible occurrence of nuclear genes that were once targeted to mitochondria. If the amitochondriate state of heteroloboseans reflects loss of mitochondrial function, it should be possible to identify nuclear genes that were derived from the alpha proteobacterial symbionts that were ancestral to mitochondria. We have taken a similar but more comprehensive approach to search for mitochondrial targeted proteins in the amitochondrial parasite Giardia lamblia. Using full genome characterizations of the diplomonad G. lamblia, we have only discovered a single gene, cpn60, which is clearly related to cpn60 genes of alpha proteobacteria. The absence of other mitochondrial-like coding regions in the Giardia genome suggests that it never harbored a mitochondrial-like organelle. Finally, we continue to explore phylogenetic relationships for parabasalids,which are deep branching eukaryotes and lack mitochondria. We have developed a rather extensive phylogeny for this group of protists with the addition of Snyderella tabogae, Calonymha grassii and Metacoronympha senta.