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Morphological evolution and development of the euglenid cytoskeleton Esson, Heather Jean


In an effort to better understand character evolution in the cytoskeleton (pellicle) of euglenid protists, I used comparative and descriptive methods to investigate the morphological diversity and development of pellicle surface patterns formed by differences in strip length at the anterior and posterior ends of the cell (strip reduction). By observing dividing Euglena gracilis cells with scanning electron microscopy (SEM) and integrating these data with previous evolutionary and developmental research, I showed that these patterns result from the semiconservative duplication and subsequent intermittent growth of pellicle strips during cytoskeletal replication and cytokinesis. Furthermore, simple changes in the developmental timing of this process (heterochrony) resulted in the diversity of posterior strip reduction patterns observed in phototrophic euglenids. This model was then used to interpret the results of two studies describing pellicle surface patterns in other photosynthetic taxa. The first was a morphological description of the complex linear pattern of posterior reduction in the benthic marine phototroph, Euglena obtusa. The second was an investigation of the evolution of bilaterally symmetrical, “clustered” strip reduction patterns in the rigid genus Phacus, examined in the context of maximum likelihood (ML) and Bayesian phylogenetic analyses of combined nuclear small subunit and partial large subunit ribosomal genes (SSU rDNA and LSU rDNA, respectively). These studies, taken together, show that strip length and other pellicle characters (such as pore placement) are strongly influenced by age and perhaps other developmental factors (such as parental strip identity and cell polarity), but the underlying genetics and molecular biology of these factors are completely unknown. Finally, SEM was used for the first time to describe prearticular strip projections, a pellicle character that has been extensively studied using transmission electron microscopy (TEM). The novel character state revealed by this study shows that the diversity of this pellicle character is still poorly understood. The structural complexity of the euglenid pellicle and the developmental and evolutionary processes that resulted in its astonishing diversity could make it an ideal model system for studying cytoskeletal evolution and development once a robust genetic research framework is constructed.

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