Ultrastructure of Septa in Dimargaris cristalligena R. K. Benjamin

Abstract

The fine structure of the septum in the vegetative hypha of the mycoparasite Dimargaris cristalligena is described. The cross-wall, continuous with the inner electron-lucent layer of the hyphal wall, bears a central pore with a flared margin occupied by a biumbonate electron-dense plug. A globose body, surrounded by a zone of ribosome-free cytoplasm, is situated next to the septal pore on each side of the cross-wall. In the neck of the haustorium, a globose body is normally observed only on that side of the septal pore next to the appressorium. Globose bodies appear structureless ; they are electron-dense when fixed in potassium permanganate and of variable electron density in aldehyde-osmium fixations. Cytoplasmic continuity is maintained through the septal pore by the plasmalemma. In the sporangiophore and sporiferous branchlets, the septal plug bears an upper globose protuberance and a lower obconic one. The hyphal septum in Tiegherniomyces californicus is ultrastructurally similar to that of D. cristalligena. The possible functional and taxonomic significance of such septa is indicated. I N T R O D U C T I O NDimargaris cristalligena, like other species in the genus, is parasitic on members of the Mucorales. Characteristically, it produces two-spored merosporangia and the hyphae are regularly septate. Liberated sporangiospores germinate on nutrient agar and the mycelium may make weak saprophytic growth in the absence of the host. Complex septa are produced in the mycelium, typically having a cross-wall with a flared central pore occluded by a plug, and usually with a spherical body on each side of the plug (Benjamin, 1959). Although the distinctive form of the septum found in the vegetative hyphae, as observed in the light microscope, is used as a diagnostic character of the Dimargaritaceae, the fine structure of such septa has not been described. Ultrastructural studies of septa (e.g. Moore & McAlear, 1962 ;Moore, 1965 ;MOSS, 1975 ;Khan, 1976) have provided substantial information supporting taxonomic relationships. The aim of this paper is to describe the fine structure of the septum of the vegetative hypha of D. cristalligena, with notes on the sporangiophore septum and on a related species Tieghenziomyces californicus, for comparison with other known forms of septa. METHODS Organisms

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