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Studies on the Morphology and Evolution of 'Orphan' Eukaryotes
Heiss, Aaron A.
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Most living eukaryotes are currently classified into one of five or six ‘supergroups’, which are in turn often divided between two assemblages: ‘unikonts’ and ‘bikonts’. This thesis explores the cytoskeletal morphology and phylogeny of three lineages that do not belong to any supergroup: ancyromonads, apusomonads, and breviates, likely relatives of supergroups Opisthokonta and Amoebozoa. It also investigates the phylogeny of malawimonads (basal members of supergroup Excavata) and collodictyonids (another unaffiliated lineage). Serial-section transmission electron microscopy was used to model the flagellar apparatus cytoskeletons of the ancyromonad Ancyromonas sigmoides, the breviate Breviata anathema, and the apusomonad Thecamonas trahens. Each has two main posterior microtubular roots and at least one anterior root (two in Ancyromonas). All three possess splitting posterior right microtubular roots and supernumerary singlets, features also characteristic of basal members of the supergroup Excavata (‘typical excavates’). One peripheral microtubule system in Ancyromonas, and the ‘right ribbon’ in Thecamonas, are likely homologous to dorsal fans in Breviata and ‘typical excavates’, and to the ‘r2’ root of myxogastrid Amoebozoa. One of the branches of the splitting root in Breviata and Thecamonas joins the right and intermediate roots, similarly to some myxogastrids. This implies that myxogastrids, and not the simpler pelobionts, represent the ancestral state for Amoebozoa. A phylogenomic analysis was performed focussing on apusomonads breviates, ancyromonads, and the problematic ‘typical excavate’ malawimonads, based on new transcriptomic data from Ancyromonas and an undescribed malawimonad. Rapid-site- removal analyses recover the ‘unikont’/‘bikont’ partition, and do not support the previously demonstrated affiliation between breviates and the ‘unikont’ supergroup Amoebozoa. Specifically, they group apusomonads with the ‘unikont’ supergroup Opisthokonta, and ancyromonads with breviates. Taxon-removal analyses group ancyromonads, breviates, and apusomonads together. Most analyses group malawimonads (perhaps with collodictyonids, another problematic group) between ‘unikonts’ and (other) ‘bikonts’, while other excavates are in a basal position amongst other ‘bikonts’. Combining these morphological and phylogenetic results suggests that splitting right roots, supernumerary intermediate singlets, and dorsal fans are found in multiple ‘basal’ lineages in both ‘unikont’ and ‘bikont’ portions of the eukaryotic tree, are likely characters of the last common ancestor of most or all living eukaryotes.