| The pivotal flagellate protists Mesostigma viride (Viridiplantae), Apusomonas proboscidea (Apusomonadidae), and three novel Stramenopiles were investigated using molecular phylogenetics and/or ultrastructure in order to understand their evolution and ecology.; Alpha- and beta-tubulin gene sequences obtained from representatives of Prasinophytae and charophycean green algae and M. viride were analyzed with a goal of resolving the controversial phylogenetic position of this green flagellate. The results supported the hypothesis that Mesostigma is an early diverging member of Streptophyta (=charophycean green algae+Embryophyta) and rejected an alternative hypothesis that Mesostigma is an earliest branch in Viridiplantae (=green algae+Embryophyta). These results support the relevance of Mesostigma as a model system for studies of the origin of Streptophyta.; The phylogenetic position of the heterotrophic biflagellate A. proboscidea was inferred based on six nuclear-encoded genes: Alpha-tubulin, beta-tubulin, actin, Hsp90, SSU rRNA, and LSU rRNA. To increase taxon richness of the study, molecular sequence data from representatives of several major eukaryote lineages were also obtained. On balance, the results suggested that A. proboscidea is sister to Opisthokonta, if the eukaryote root does not lie between them. These results are consistent with existing morphological data that Opisthokonta (which are characterized by a single flagellum) descended from biflagellate ancestors. This study adds to a body of data that case doubt on a recent hypothesis that all eukaryotes are divided into two clades-'unikonts' (Amoebozoa and Opisthokonta), and 'bikonts', (all others).; Three novel heterotrophic, flagellate Stramenop---Apokia fantasma (Chrysophyceae), Filos agilis (Bicosoecida), and Nanos amicus (Bicosoecida)---were isolated from acidic lakes. While A. fantasma is an obligate heterotroph occurring as biflagellate cells held within a common mucilage, SSU rRNA phylogeny suggested that the flagellate is secondarily derived from mixotrophic ancestors. Consistent with this result, a leucoplast-like spherical structure was observed in the perinuclear region of A. fantasma. Tiny uniflagellates F. agilis and N. amicus were separately isolated from the mucilage of A. fantasma in geographically distinct freshwaters. The uniflagellates appeared to feed on A. fantasma mucilage or organic particles associated with the mucilage, thereby benefiting from the symbiosis. This study suggests that protist diversity is higher in acidic environments than previously realized. |
