The Ultrastructure and Development of Sporangia In Gilbertella Persicaria

SUMMARY

The ultrastructural features of sporogenesis in the mucoraceous fungus Gilbertella persicaria are illustrated and described, with emphasis on cytokinesis and spore wall formation. During cleavage, the principal structural changes involve pattern transformations of protoplasmic membranes. Endoplasmic reticulum (ER) is one of the most changeable membrane components, transforming from a complex fenestrated and branching form to a simpler sheet-like form during the course of cleavage.

During precleavage, small vesicles are formed, apparently from special cisternae. The disappearance of these initial vesicles coincides with the appearance of cleavage vesicles. Cleavage vesicles are distinguished by the presence of granules on the inner surface of the vesicle membrane. Prior to cleavage, the nuclei, ER, and cleavage vesicles form an inter-associated system. Cleavage is initiated endogenously by the coalescence of cleavage vesicles to form a ramifying tubular cleavage apparatus. The cleavage apparatus demarcates the boundaries of potential spore initials. Lateral expansion of elements of the cleavage apparatus results in furrowlike configurations which converge to cut out spore initials as independent cells. The cleavage membrane is transformed to the plasma membrane of spore initials during late cleavage, at a time when the cleavage membrane and sporangial plasma membrane fuse. The columella is delimited by an apparatus that is structurally similar to the cleavage apparatus in the spore plasm. Columellar cleavage is completed when the spore plasm is at mid- cleavage.

The marker granules that were present around the periphery of the cleavage vesicles are found on the outer surfaces of spore plasma membranes after cleavage. The granules fuse to form a continuous spore envelope, and subsequently the spore wall is laid down centripetally. Thus, the envelope becomes the outermost spore wall layer.