UBC Theses and Dissertations

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UBC Theses and Dissertations

The phylogeny and evolution apicomplexan parasites Mathur, Varsha


Apicomplexans are a large phylum of obligate animal parasites that contain pathogens such as Plasmodium spp. (the causative agent of malaria) and Toxoplasma gondii. While these medically relevant apicomplexans are the subject of extensive research, the bulk of the diversity of the group, particularly the lineages that infect invertebrates, remain poorly studied and largely ignored in high-throughput sequencing surveys. In this dissertation, I show that these groups are critical to gaining insights into the origins and evolution of the Apicomplexa. I begin by examining the diversity and inferred ecology of the enigmatic apicomplexan-related lineages (ARLs), and show that ARL-V is highly abundant in environmental surveys, and is tightly associated with coral tissue and mucus, suggesting that it represents a core symbiont of coral. In the following chapters, using methods of single-cell transcriptomics, I sequenced the transcriptomes of 15 invertebrate-infecting apicomplexans. Using this dataset, I constructed a robust and taxon-rich multi-gene apicomplexan phylogeny that resolves the deep phylogenetic relationships within the group, and also form a new class of apicomplexans, the Marosporida, that is sister to the Hematozoa and Coccidia. Most unexpectedly, in Chapter 2, I show that certain taxa previously classified as apicomplexans, actually represent convergently evolved animal parasites, suggesting that apicomplexan-like parasites have evolved at least four times independently. In Chapter 3, I examine the presence and function of apicoplasts (remnant plastids) across the diversity of the group using whole genome shotgun sequencing (WGS), and find that the Marosporida contain the smallest, most AT-rich, and gene poor apicoplast genomes sequenced to date. I also present the first evidence of plastids in the gregarines, and show that archigregarines retain the canonical apicomplexan plastid metabolism, whereas only one clade of marine eugregarines retains plastids that solely carry out type II fatty acid biosynthesis. Lastly in Chapter 4, I reconstruct the mitochondrial metabolism in the gregarines and squirmids, and find that eugregarines contain highly reduced respiratory chains, suggesting that they have lost their mitochondrial genomes, and possess limited energy metabolism. Altogether, the data presented here, illustrates the significance of invertebrate-infecting apicomplexans in illuminating the early evolution of the apicomplexans and myzozoans.

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