The fine structure of ascospore formation in Ceratocystis fimbriate

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Abstract
Ascospore formation in C. fimbriata Ell. & Halst. takes place in the eight-nucleate ascus. It is preceded by the appearance of discrete vesicles in the ascus which coalesce to form individual membrane fragments. These membrane fragments elongate to form long continuous strands of double membranes which become the ascospore-delimiting membranes. Ascospores are delimited in one of two ways within the ascus. In one way the strands of double membranes seem to form a sac around each of the nuclei, which eventually closes to form the ascospore. In the second method the strands of double membranes form a large ascus vesicle which encloses the eight nuclei and most of the ascus cytoplasm. The ascospores are then delimited by the formation of lobes on the ascus vesicle. Each of the newly delimited ascospores appears to be paired on one side with another ascospore. After spore delimitation is complete, the ascospore wall is laid down between the ascospore-delimiting membranes. After the walls are differentiated, a brim forms on each side of the flattened area of contact between the paired ascospores. On maturity the ascus breaks down and single, brimmed ascospores are released.