THE STRUCTURE OF THE SPORES OF GIGASPORA MARGARITA

Abstract

SUMMARYLight and electron microscopy have been used to study the emergence through the spore wall and subsequent growth and development of the germ‐tube of Gigaspora margarita. Spore‐wall penetration by the germ‐tube is unusual as two different mechanisms seem to be operating, one for the inner spore‐wall layers and another for the outer layer. As the germ‐tube develops a primary wall layer is formed, and this can be traced back to its origin as a layer continuous with the thickened region of the innermost spore‐wall layer. In the maturing germ‐tube a highly osmiophilic secondary wall is deposited and this may be the wall layer that persists when the fungus penetrates a host root and sets up a mycorrhizal association.In common with other fungi that have been studied, the ultrastructural organization found in G. margarita can be categorized into approximately three zones: the apical zone, the subapical zone and the zone of vacuolation. Other features of the cytoplasmic organization are also discussed including the possible significance of bacteria‐like organisms, membrane‐bound crystals, osmiophilic granules and glycogen particles. Features of the culture‐grown germ‐tubes are also compared with those of host‐associated stages of the fungus reported in the literature.