The biology and systematics of myxozoan parasites of fish and oligochaetes from Lake Sasajewun, Algonquin Park, Ontario

Since the discovery of the 2-host life cycle of myxozoans by Wolf and Markiw in 1984, numerous studies have contributed to the knowledge of myxozoan parasites, and at the same time, have raised even more questions. Most of these studies have only dealt with the experimental transmission of these parasites under laboratory conditions. In my dissertation, I have attempted to address some of the questions by looking at different aspects of these enigmatic parasites in a natural ecological setting.;To fully document myxosporeans in Lake Sasajewun, a survey of these parasites of fish was conducted which resulted in the description of 2 new species. An intensive survey of oligochaete hosts of actinosporeans in the lake revealed that the fauna consisted of 19 species. Upon examining more than 14,000 worms for actinosporeans, only 146 worms were found to be releasing spores. Despite a prevalence of only 1.0%, 22 new 'species' of actinosporeans of 8 collective groups were described. The number of actinosporean 'species' is less than half the number of myxosporean species from the same lake, suggesting that direct transmission might occur among fish. Water-borne actinosporean spores peaked when water temperatures of the lake ranged from 18 to 24 C, which coincided with the feeding and growing season of larval fish.;The molecular variation of 18 myxozoans from Lake Sasajewun was analyzed by riboprinting, revealing a close relationship between Myxobolus pendula and Myxobolus pellicides, and between triactinomyxon 'C' and Triactinomyxon ignotum. Parsimony analysis of riboprint data lent further support to the 2-host life cycle hypothesis. The genetic diversity of selected myxozoans was further investigated by sequence comparisons of the 18S rRNA gene. The phylogenetic hypothesis generated from the sequence data in this study and those available from GenBank also supported the alternating life cycle hypothesis.;Phylogenetic analyses of the myxozoan species with known alternating life cycles, using morphological and developmental characters from myxosporean and actinosporean phases, were conducted through a total evidence approach. The analysis revealed that the suborder Variisporina and the family Myxobolidae were not monophyletic groups and that myxozoans were subject to thorough systematic revision.