Evolution and comparative morphology of the euglenophyte plastid
Brown, Patrick Jerry Paul
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My doctoral research centered on understanding the evolution of the euglenophyte protists, with special attention paid to their plastids. The euglenophytes are a widely distributed group of euglenid protists that have acquired a chloroplast via secondary symbiogenesis. The goals of my research were to 1) test the efficacy of plastid morphological and ultrastructural characters in phylogenetic analysis; 2) understand the process of plastid development and partitioning in the euglenophytes; 3) to use a plastidencoded protein gene to determine a euglenophyte phylogeny; and 4) to perform a multigene analysis to uncover clues about the origins of the euglenophyte plastid. My work began with an alpha-taxonomic study that redefined the rare euglenophyte Euglena rustica. This work not only validly circumscribed the species, but also noted novel features of its habitat, cyclic migration habits, and cellular biology. This was followed by a study of plastid morphology and development in a number of diverse euglenophytes. The results of this study showed that the plastids of euglenophytes undergo drastic changes in morphology and ultrastructure over the course of a single cell division cycle. I concluded that there are four main classes of plastid development and partitioning in the euglenophytes, and that the class a given species will use is dependant on its interphase plastid morphology and the rigidity of the cell. The discovery of the class IV partitioning strategy in which cells with only one or very few plastids fragment their plastids prior to cell division was very significant. This partitioning strategy is unique to the euglenophytes and is correlated to the degree of ‘euglenoid movement’ in a given species. An analysis of the PsaA gene from numerous euglenophytes had three main results: 1) the Eutreptiales are basal and paraphyletic; 2) there is some justification for a relationship between Euglena anabaena and E. gracilis; and 3) plastid genes are unsatisfactory for phylogentic analysis of the euglenophytes. Finally I performed a multi-gene analysis of the PsaA, RbcL, and 16S rDNA genes from all major groups of eukaryotic algae. This showed that the euglenophyte plastid likely has its origin from within the green algal class Prasinophyceae.
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