ULTRASTRUCTURE OF ASCUS ARRANGEMENT AND ASCOSPORE DEVELOPMENT IN OPHIOSTOMA-SETICOLLE

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