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Molecular evolution, systematics and ecology of microsporidia from fishes and crustaceans Brown, Amanda May Vivian

Abstract

Microsporidia are unicellular, intracellular parasites once considered primitive protists without mitochondria, but recently shown by DNA analysis to be related to Fungi. These pathogens have undergone evolution characterized by dramatic losses of genome complexity during adaptation and speciation within host cells. Microsporidian infections can cause mortality in fishes and crustaceans and significant commercial losses in wild, hatchery and fish farm stocks. This study examined microsporidia in salmon, trout, Atlantic cod, Pacific cod, lingcod, walleye pollock, haddock, sablefish, Pacific tomcod, smooth pink shrimp, and a copepod, from marine and freshwater habitats on the east and west coasts of Canada and elsewhere. The purpose was to examine problems in morphological and molecular classification, identify relationships in light of taxonomic and host-parasite co-evolution hypotheses, and combine these data towards description of new species. Ribosomal RNA (rDNA), elongation factor-1 alpha and RNA polymerase II genes were amplified. Phylogenies were reconstructed and tested statistically. Three bioinformatics software (Java) programs were developed to aid DNA analysis. Light and transmission electron microscopy (TEM) were used to characterize spores, developmental stages, and pathogenesis. Among 26 isolates of Loma salmonae, most genetic diversity occurred in farms in California, Colorado and Chile (up to 0.66 % rDNA difference), while wild isolates from British Columbia were invariant. Ten other Loma species (1 to 8 isolates each) grouped in phylogenetic clusters often corresponding to species or host. Five new Loma species were described based on developmental timing, spore and xenoma features. Intraspecific polymorphism caused low genetic resolution of some species. Loma species displayed high evolutionary and developmental phenotypic plasticity relative to genetic variance. Data showed host-parasite co-evolution in family Gadidae, and separated previously synonymized taxa (L. branchialis and L. morhua, L. salmonae and Loma sp. in brook trout). Thelohania butleri from shrimp was closely related to microsporidians in marine decapods and grouped apart from other Thelohania species, suggesting this genus needs revision. A new genus and species was described from a copepod, based on production of a mucocalyx and 3 distinct spore forms - features suggesting a possible secondary host. Overall, host and ecology better predicted relationships than did morphology, emphasizing the importance of plasticity.

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