Ultrastructural analysis of basidiosporogenesis in Panellus stypticus

Abstract

The ultrastructure of events in basidiosporogenesis in Panellus stypticus was examined using conventional, aqueous-based fixation procedures and freeze substitution fixation following plunge freezing in liquid propane. Freeze substitution was superior in preserving cytological features and in retaining cell wall and extracellular materials. Synapsis, all stages of meiosis I (including prophase, metaphase, anaphase, and telophase), and prophase of meiosis II were observed. The nuclear envelope breaks down during meiosis I, temporarily reforms during interphase, and is at least partially broken down during meiosis II. Many stages of spore development, including sterigma initiation, sterigma elongation, organelle translocation, and nuclear migration, were observed. Spindle pole bodies with microtubule arrays were associated with nuclear migration into developing spores. Analysis of hymenial cells with gold-tagged lectins and enzymes revealed an α-amylase positive outer cell wall layer specific to basidiospores. Only after basidiospore release were surfaces of sterigmata and basidia similarly labeled. All cell walls observed were positive for wheat germ agglutinin, indicating the presence of chitin. Septa-delimiting basidiospores from sterigmata were heavily labeled with wheat germ agglutinin. This is the first investigation of basidiosporogenesis in a homobasidiomycete preserved for transmission electron microscopy by rapid freezing and freeze substitution. Key words: fungal cell walls, lectins, gold labeling, meiosis, rapid freezing, transmission electron microscopy.