Characterization of Microtubule Organizing Centers in the genus Protostelium, Including Evolutionary Implications

Microtubule organizing centers (MTOCs) are cellular regions of microtubule nucleation. The best known MTOCs are those associated with the centrosome, but several non-centrosomal MTOCs are known in eukaryotes, especially in land plants. MTOCs are poorly characterized across the breadth of amoebozoan diversity, but are well-known in certain amoebozoan lineages, including the genus of protosteloid slime molds Protostelium. The structure of the MTOC is known for two non-ciliated species, P. nocturnum and P. mycophaga, as well as P. aurantium, which can reversibly become ciliated under appropriate conditions. P. nocturnum and P. mycophaga have acentriolar centrosomal MTOCs while P. aurantium has a centriole-bearing pro-kinetid that differentiates into a kinetid when the cell becomes ciliated. It was previously thought that the MTOCs of P. mycophaga and P. nocturnum were homologous to each other, and were derived from a structure reminiscent of the kinetid of P. aurantium, but recent changes in our understanding of the group’s phylogeny, as well as the realization that most isolates of P. aurantium cannot become ciliated, have called this hypothesis into question. In this thesis, a new strain of P. aurantium was isolated. This strain, which was unable to produce cilia when isolated, was characterized ultrastructurally and found to have an MTOC typical of non-ciliated Protostelium spp. After ultrastructural work was complete, ciliated cells were unexpectedly found in one culture of the new isolate. The significance of these findings, and their implications for the evolutionary history of Protostelium, are discussed