ANALYSIS OF THE ROLE OF THE SPITZENKORPER IN FUNGAL MORPHOGENESIS BY COMPUTER-SIMULATION OF APICAL BRANCHING IN ASPERGILLUS-NIGER

High-resolution video microscopy, image analysis, and computer simulat ion were used to study the role of the Spitzenkorper (Spk) in apical b ranching of ramosa-1, a temperature-sensitive mutant of Aspergillus ni ger. A shift to the restrictive temperature led to a cytoplasmic contr action that destabilized the Sph, causing its disappearance, After a s hort transition period, new Spk appeared where the two incipient apica l branches emerged, Changes in cell shape, growth rate, and Spk positi on were recorded and transferred to the FUNGUS SIMULATOR program to te st the hypothesis that the Spk functions as a vesicle supply center (V SC), The simulation faithfully duplicated the elongation of the main h ypha and the two epical branches, Elongating hyphae exhibited the grow th pattern described by the hyphoid equation, During the transition ph ase, when no Spk was visible, the growth pattern was nonhyphoid, with consecutive periods of isometric and asymmetric expansion; the apex be came enlarged and blunt before the apical branches emerged, Video micr oscopy images suggested that the branch Spk were formed anew by gradua l condensation of vesicle clouds, Simulation exercises where the VSC w as split into two new VSCs failed to produce realistic shapes, thus su pporting the notion that the branch Spk did not originate by division of the original Spk, The best computer simulation of apical branching morphogenesis included simulations of the ontogeny of branch Spk via c ondensation of vesicle clouds. This study supports the hypothesis that the Spk plays a major role in hyphal morphogenesis by operating as a VSC - i.e., by regulating the traffic of wall-building vesicles in the manner predicted by the hyphoid model.