Abstract
Ceratocystis ulmi, the Ascomycete responsible for Dutch elm disease, may sporulate by means of a distinctive structure, the synnema, common in nature in the tunnels of elm bark beetles. Developmental stages of this structure and its spores were examined by light microscopy, by conventional transmission electron microscopy of thin sections and freeze-etch replicas, and by scanning electron microscopy of whole spores and fruiting structures. The synnema is a tight bundle of darkly pigmented hyphae growing erect from the substrate and terminated by a mass of colorless, wet spores. A layer of slime covering the hyphal bundle and random cross-connections between parallel hyphae appear to function in stabilization of the structure. Organelles typical of Ascomycetes fill the spores, vegetative hyphae, and young synnemal hyphae. However, in mature synnemal hyphae, the cytoplasmic contents degenerate, leaving only membranous vesicles in the partially collapsed hyphae. The spores contain large lipid droplets not found in either vegetative or synnemal hyphae indicating some differences in metabolism of spores and hyphae. Most synnemal spores form on sympodulae, but some spores form on intrahyphal hyphae.
Published Version
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