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

Abstract

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.