Ultrastructure of Dormant, Converted, and Germinating Oospores of Pythium Ultimum

The ultrastructural aspects of the conversion of constitutively dormant oospores of Pythium ultimum to thin-walled germinable oospores and their subsequent germination were studied by electron microscopy. During conversion, the thick inner oospore wall dissolves leaving the outer oospore wall as a loose but intact layer surrounding the spore. A thin germination wall, which eventually extends as the germ tube wall, becomes apparent around the protoplast. The protoplasm of the dormant oospore is predominantly reserve cytoplasm and includes a cortical layer of lipid bodies and a dense central ooplast. Converted oospores contain a more functional protoplasm including nuclei, mitochondria, endoplasmic reticulum, dictyosomes, and unattached ribosomes. The cortical lipids disperse within the cytoplasm while the ooplast breaks into fragments which are enveloped in membranes to form the densebody or “fingerprint” vacuoles characteristic of the oomycetous fungi. After sucrose is added, germinable oospores produce single germ tubes which rupture the outer oospore and oogonial walls. Germ tube tips are laden with dictyosome-derived vesicles.