Ultrastructure of the fungus Petersenia palmariae (Oomycetes) parasitic on the alga Palmaria mollis (Rhodophyceae)

Abstract

Invasion of Palmaria mollis by the marine oomycete Petersenia palmariae begins with penetration of the cell wall and then the lumen of an outer cortex cell. The fungus in its vegetative phase lacks a wall but is separated from the host cytoplasm by the invaginated plasmalemmaof the host cell. By promoting fusion of host cells or by the dissolution of the host cells' pit plugs, the fungus is able to use a symplastic route to invade cells deep in the cortex and medulla of the host. In the process, a compound confluent host cell lumen is created. The deposition of a thick wall around the lobed fungal cell marks the beginning of holocarpic sporangium formation and is followed by great proliferation of the parasite's nuclei and concomitant diminution of nuclear and nucleolar size. The cytoplasm becomes parietal when a large central vacuole develops. Mastigonemes are formed in dilated endoplasmic reticulum, mitochondria become associated with nuclei, and cleavage begins. Flagella begin to form on paired basal bodies before cleavage is complete. Following a period of motility, zoospores retract and then resorb their flagella. The naked spores develop walls and the basal bodies persist as centrioles.