Light and electron microscopy of oospore maturation in Saprolegnia furcata: 2. Wall development

Abstract

Wall formation in maturing oospores of Saprolegnia furcata Maurizio has been studied by light and electron microscopy. Following oosphere delimitation an outer (epispore) wall is secreted by Golgi-derived vesicles. Outer wall formation continues after fertilization but ceases prior to inner (endospore) wall initiation. The inner wall of the oospore is accreted centripetally until it reaches a thickness of about 1.5 μm. Wall vesicles do not appear to be involved and only cisternae of endoplasmic reticulum lie close to the growing wall. Inner and outer walls remain separated by an electron-dense tripartite, membrane-like, layer. The appearance of the wall of the mature oospore varies with the fixative employed. Whereas the inner wall appears uniformly electron transparent in glutaraldehyde-osmic fixed oospores, both osmic acid and permanganate fixation reveals electron-dense inner and outer zones within this layer. The inner zone, also discernible in living spores, is bounded on one side by the cytoplasmic membrane and has a distinct convolute outer margin. The outer zone is bounded by the tripartite layer and has an indistinct inner margin. Permanganate fixation also reveals the fibrillar substructure of the inner wall and staining with periodic acid-silver methenamine indicates that it is rich in polysaccharides. In addition, the unusual wall structure of abnormally swollen but well-fixed oospores is described. Oospore wall development in S. furcata is compared with that in other oomycetes and the origin and nature of cell wall components is discussed.