AGGREGATION AND COLLAPSE OF FUNGAL WALL VESICLES IN HYPHAL TIPS - A MODEL FOR THE ORIGIN OF THE SPITZENKORPER

The intracellular origins of polarity and branch initiation in fungi c entre upon a localization in the supply of fungal wall constituents to specific regions on the hyphal wall. Polarity is achieved and maintai ned by accumulating secretory vesicles, prior to incorporation into th e wall, in the form of an apical body or Spitzenkorper. However, neith er the mechanisms leading to this accumulation nor the initiation of b ranching, are as yet understood. We propose a mechanism, based on expe rimental evidence, which considers the mechanical properties of the cy toskeleton in order to explain these phenomena. Cytoskeletal viscoelas tic forces are hypothesized to be responsible for biasing vesicles in their motion, and a mathematical model is derived to take these consid erations into account. We find that, as a natural consequence of the a ssumed interactions; between; vesicles and cytoskeleton, wall vesicles aggregate in a localized region close to the tip apex. These results are used to interpret the origin of the Spitzenkorper. The model also shows that an aggregation peak can collapse and give rise to two new c entres of aggregation coexisting near the tip. Wee interpret this as a mechanism for apical branching, in agreement with published observati ons. We also investigate the consequences and presumptive role of vesi cle-cytoskeleton interactions in the migration of satellite Spitzenkor per. The results of this work strongly suggest that the formation of t he Spitzenkorper and the series of dynamical events leading to hyphal branching arise as a consequence of the bias in vesicle motion resulti ng from interactions with the cytoskeleton.