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UBC Theses and Dissertations

Petersenia pollagaster (Oomycetes) : an invasive pathogen of Chondrus crispus (Rhodophyceae) Molina, Francis I.


Cytochemistry, vital staining, light- and transmission electron microscopy were employed to study the host-parasite relationship between a pathogenic oomycete and a maricultured gametophytic strain of Chondrns crispus Stackhouse (Irish moss). Inoculation of healthy algal thalli with infective fungal zoospores was performed at five temperatures and the highest infection rate was obtained at 20C. The cycle consisting of infection of healthy thalli, endobictic development, and release of zoospores is completed in 48-72 h at this temperature. The parasite initially invades cortical cells of the host and spreads to the sub-cortical and medullary tissues. It is endobiotic, simple, holccarpic, and produces heterokont zoospores which are released via a single exit tube. Cytochemical staining procedures employing Calcofluor White, Congo Red, and Zinc Chloriodide revealed the presence of cellulose in sporangial walls. The pathogen is assigned to Petersenia pollagaster (Petersen) Sparrow. Four fluorochroraes were initially screened for fluorescence differentiation between living and dead host cells. The most consistent results were obtained with 4',6-diamidino-2-phenylindole (DAPI). Incubation of newly inoculated host tissue in this fluorochrome resulted in the fluorescence of host cells around infection sites, showing extensive host tissue damage. Ultrastructural studies revealed that zoospores retract their flagella during encystment cn the host alga. They soon become invested with a thin wall and a fuzzy cell coat which may have a role in host recognition and attachment. The development is mainly intracellular. The parasite initially develops within the host as a protoplast. Its wall-less vegetative thallus proliferates via a symplastic route and causes the fusion of host cells through the removal of pit plugs. Damage to host tissue is minimal during the early stages of development and suggests the parasite's need for a living host. Mobilization of floridean starch grains in the host cytoplasm is indicated by their apparent dissolution. Death of host cells, characterized by disintegration of the plasmalerama, appears to trigger zcosporcgenesis. Cleavage of the sporogenic cytoplasm is accomplished by the fusion of Golgi-derived cisternae and small vacuoles. Formation of zoospores is completed within the sporangium. The death of host cells at infection sites and the demise of some zoospores inside the algal thallus implies a hypersensitive host response.

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