Ultrastructural observations and a computer model of the helicoidal appearance of the spore wall of Glomus geosporum

Using transmission electron microscopy, the spore wall of Glomus geosporum was shown to possess two layers. The outer layer is thin and evanescent, whilst the inner layer is thick and laminated. A plug or septum which delimits the spore contents from the subtending hypha is closely associated with the basal lamina of the inner layer. Each lamina within the wall displayed an arcuate pattern of microfibrils, typical of cell walls having a helicoidal architectural substructure. During germination, an additional inner wall layer was deposited continuous with the wall of the emerging germ-tube. The ultrastructural appearance of the laminate layer of the spore wall of G. geosporum was reproduced by computer using a simple helicoidal model, and the effect of varying certain parameters was examined. Initially, the patterns obtained by computer modelling did not conform exactly to the appearance of sections of spore walls of G. geosporum. A closer fit was obtained when a cyclical factor was introduced, acting on the angle of rotation between successive layers of microfibrils, which suggests that external rhythmical factors may influence the spore wall structure. Certain combinations of factors resulted in the disappearance of the typical arced pattern. It is concluded that in spores of other glomalean fungi, such as those of G. mosseae, a helicoidal structure may be present but difficult to demonstrate by ultrathin sectioning.