Scanning electron microscopy of surface ultrastructure changes during meiosporangium maturation and meiospore liberation in the aquatic fungus Allomycpes arbuscula.

Alterations in wall ultrastructure accompanying resistant sporangium maturation and meiospore liberation in Allomyces arbuscula were examined by scanning electron microscopy. Three discrete wall layers were identified, each of which underwent marked changes during processes leading to zoospore release. The outermost wall layer, the hyphal sheath continuous with the hypha, was physically altered during the maturation process preparatory to induction and release of meiospores. The integrity of this wall layer was broken, and it was no longer closely juxtaposed to the heavy pitted wall layer that lay beneath it. A fibrillar matrix seemed to cement the two layers to one another before this desiccation. A single, raised, longitudinal dehiscence ridge on each meiosporangium appeared to be a structurally differentiated region of the pitted wall layer at which sporangium rupture occurred to permit emergence of the protoplast. By its thickness the pitted wall layer was likely to provide mechanical rigidity to the meiosporangium. Beneath the pitted wall layer, another thin, flexible wall layer surrounded the protoplast. From this structure, a single exit papilla was cleaved at the apical region to effect the release of meiospores from the protruding protoplast. Thus a sequence of structural changes in well-differentiated multiple wall layers is implicated in the sporulation process in this organism.