Ultrastructure of ascus arrangement and ascospore development in Ophiostoma seticolle

Abstract

Species in Ceratocystis and Ophiostoma have unusual ascospore morphology. Ascospores can have hat-shaped, elongate, inequilateral, needle or pillow-shaped forms. The ultrastructure and development of pillow-shaped ascospores, such as those occurring in O. seticolle, have never been examined critically. In O. seticolle, ascospore development occurs in irregularly shaped, thin-walled asci present in young ascomata. Each ascus contains eight nuclei surrounded by delimiting membranes, which lead to ascospores having at least three wall layers. The outermost wall layer is characterized by the formation of ridges that give the ascospores their pillow-shaped appearance. Ascospores of O. seticolle are released in a slimy matrix, and what appear to be sheaths surrounding the ascospores, reported in most light microscopic studies, are actually multilayered ascospore walls.

Key Words:

• ascoma
• ascospores
• centrum
• Ceratocystis
• Ophiostoma
• pillow-shaped sheath