The ascus vesicle

Ultrastructural observations on ascosporogenesis in Ciboria betulae (Helotiales : Sclerotiniaceae)

The ultrastructure of ascospore ontogeny including spore delimitation and spore wall formation is examined in Ciboria betulae. A peripheral delimiting cisterna is derived from the ascus plasmalemma apparently by the self-assembly of subunits in the cytosol. The cisterna is discontinuous and delimits each of eight incipient ascospores in the ascus. The particular, verrucose ascospore ornamentation in C. betulae appears early during primary wall formation. The electron-lucent primary wall differentiates into an endospore of finely fibrillar texture and a two-layered, fibrillar epispore. The ultrastructural changes of ascus epiplasm, sporoplasm, and spore wall are described and compared with previous studies on Ascomycetes. Key words: ascosporogenesis, ascospore ontogeny, Ciboria betulae, ascospore wall differentiation.

Ultrastructure of ascosporogenesis in Nannizzia gypsea

Ascosporogenesis in Nannizzia gypsea was studied by electron microscopy. Development of ascospores began with the formation of an ascus vesicle composed of two paired unit membranes. Myelin figures consisting of coiled or concentric membranes were regularly connected with the growing ascus vesicle. Both the ascus vesicle and the myelin figures possessed an electron-dense line between paired membranes, and both were stained by the periodic acid-silver methenamine technique. Invagination of the ascus vesicle about the haploid nuclei resulted in eight uninucleate prospores bounded by two concentric membranes. Spore wall material was deposited between the two membranes of the prospores, and deposition was greatest in areas of the wall overlying stacked elements of endoplasmic reticulum. A single myelin figure surrounded by a polysaccharide halo was observed in the spore.