Evolutionary morphology of the euglenid pellicle
Leander, Brian Scott
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My doctoral research addressed questions about the evolutionary histories of euglenids, which are a diverse group of free-living flagellates that diverged early in eukaryotic history. This research represented one of the first attempts to integrate a large morphological data set for unicellular organisms into a molecular context. My main objectives were (a) to document evolutionary trends in characters associated with their cell cortex or pellicle, (b) to accrue empirical support for "adaptative", "exaptative", and "nonaptative" explanations of pellicular diversity, and (c) to develop an accurate taxonomic framework for interpreting euglenid phylogeny. Most of this work focused on the diversity of pellicle strips, which are long proteinaceous structures that articulate along their lateral margins and subtend the plasma membrane. The characteristics of different pellicles are associated with different modes of nutrition and locomotion. Using electron microscopy I identified a rich source of novel information about euglenid morphology that has implications for both developmental biology and paleontology. An independent data set of nuclear small subunit ribosomal DNA sequences was generated in parallel. I studied evolutionary trends associated with the pellicle using cladistic methods on the morphological, molecular, and combined data. A pilot study demonstrated a high degree of concordance between the morphological and 18S rDNA data sets and, hence, the utility of pellicle morphology for inferring phylogenetic relationships of euglenids. In order to address large-scale trends within the group, both the morphological and 18S rDNA data sets were expanded. The tree topology resulting from a "total evidence" analysis formed the template for a synthetic tree that took into account any conflicting results derived from the partitioned data sets. I parsimoniously mapped pellicle character states onto the synthetic tree in order (a) to convert many of my earlier primary homology statements (inferences made independently from a cladogram) into secondary homology statements (inferences made from the distribution of character states on a cladogram) and (b) to infer ancestral-derived polarities for specific character state transformations. This research not only documented idiosyncratic patterns of morphological evolution but enabled me to identify which pellicle character states are the best candidates for apomorphy-based taxon definitions within an updated classification for euglenids.