From Cap to Collar: Ontogeny of the Endocytic Collar in Neurospora crassa

Endocytosis in filamentous fungi is spatially restricted to a subapical zone known as the endocytic collar, which plays essential roles in membrane recycling and the maintenance of polarized growth. In this study, we investigated the ontogeny of the endocytic collar in <i>Neurospora crassa</i> by tracking fimbrin-labeled endocytic patches using confocal microscopy during conidial germination, hyphal branching, and regeneration following mechanical injury. We consistently observed an initial accumulation of endocytic patches at the hyphal tip, forming an apical cap, which later reorganized into a subapical collar. This transition was correlated with a significant increase in elongation rate and the appearance of a Spitzenkörper, indicating a link between exocytosis and collar positioning. Although this correlation is robust, our data do not establish causality; rather, collar formation appears to occur after surpassing a critical elongation. Our findings suggest that exocytosis displaces endocytosis from the apex, resulting in the formation of the collar, which is not required for the establishment of polarized growth but is essential for its maintenance. These results support the development of a unified model of collar formation in filamentous fungi and provide new insight into the spatial coordination between endocytic and exocytic processes during hyphal development.