Electron microscopic study of oospore maturation and germination in an emasculate isolate of Saprolegnia ferax. 3. Changes in organelle status and associations

Abstract

Changes in the structure of the main cytoplasmic organelles (excluding wall and nuclei) during the formation, maturation, and germination of Saprolegnia ferax oospores have been followed by transmission electron microscopy and their status assessed by stereologic analysis of micrographs. During oosphere differentiation there is a steady accumulation of lipid and dense-body (lipo-phospho glucan complexes) reserves; at the same time the overall status quo of other organelles is maintained. Oospore maturation is associated with the coalescence of dense-body vesicles, partial utilization of neutral lipid reserves, and dramatic decline in most other cell organelles. This is preceded by a marked increase in lipid-associated microbodies (glyoxysomes) which appear to develop from the fragmentation of cisternae of smooth endoplasmic reticulum. At the onset of germination the inner oospore wall is rapidly digested and normal levels of cytoplasmic organelles is restored. The neutral lipid fraction progressively declines with half the reserves being utilized before germ tube emergence. The osmiophilic dense-body vesicle derived ooplast matrix disintegrates and dense-bodylike granules are released into the expanding central vacuole and peripheral cytoplasmic vesicles, which eventually become associated with cup-shaped mitochondria. Unusual reticular bodies (mitochondria), crystalline aggregates (virus particles), and multivesicular bodies are also described in the cytoplasm of germinating spores. The results are discussed in relation to the possible metabolic pathways associated with the utilization of endogenous reserves.