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Contributions to the ultrastructural diversity and molecular phylogeny of phagotrophic euglenids and their episymbionts Breglia, Susana Alicia


The Euglenida is a diverse group of single celled eukaryotes with modes of nutrition that include phagotrophy, osmotrophy, and phototrophy. Phototrophic members of the group have attracted the most attention from previous researchers, and some species (e.g., Euglena gracilis) have become models in cell biology research. Phagotrophic euglenids, by contrast, are difficult to cultivate and manipulate so are severely underrepresented in culture collections, comparative ultrastructural studies, and molecular phylogenetic studies. Species discovery and the comparative ultrastructure of phagotrophic euglenids within a phylogenetic context were the main aims of this thesis. These data are essential for a comprehensive knowledge of the overall diversity and evolutionary history of euglenids as a whole, as well as for a better understanding of the relationships with their closest euglenozoan relatives, the Kinetoplastida and the Diplonemida. I generated DNA sequences of heat shock protein 90 and small subunit (SSU) rRNA genes from several different species of phagotrophic euglenids in order to evaluate some of the deepest branches in the phylogeny of euglenozoans, especially the phylogenetic position of Petalomonas cantuscygni. This species has a set of morphological features that are intermediate between kinetoplastids and euglenids (e.g., pellicle strips and kinetoplast-like mitochondrial inclusions). I also characterized the ultrastructure, feeding behaviour, and phylogenetic position of Heteronema scaphurum, a phagotrophic euglenid that feeds on green algal prey and is equipped with a distinctive “cytoproct” or cell anus. My explorations in low oxygen marine sediments led me to discover and characterize a novel lineage of euglenozoans, the “Symbiontida”. Members of this group formed intimate symbiotic relationships with at least two distinct types of epibiotic bacteria: rod-shaped epsilon-proteobacteria and spherical-shaped verrucomicrobia. I was able to show, using electron microscopy, that the verrucomicrobial symbionts were capable of evasive sporulation using a conspicuous extrusive apparatus that consisted of a thread tightly wound around a central core of DNA. The highly similar episymbionts reported previously on a group of ciliates led to questions about host transfer and the convergent evolution of extrusive organelles across the tree of eukaryotes.

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