Diversity and dynamics of the Spitzenkorper in growing hyphal tips of higher fungi

The Spitzenkorper, located in the apex of growing hyphae of septate fungi, has been portrayed previously as a spheroid complex containing a cluster of apical (secretory) vesicles which sometimes encloses a differentiated core area. With the aid of computer-enhanced video microscopy and phase-contrast optics, we studied 32 fungi in the Ascomycetes, Deuteromycetes, Hyphomycetes, Basidiomycetes, and Agonomycetes. The Spitzenkorper appeared as a highly dynamic and pleomorphic multicomponent complex capable of changing shape, size, and position within the hyphal apex during growth. The main theme of this study is to demonstrate two kinds of morphological diversity/variation in Spitzenkorper from diverse fungi: (a) inherent diversity-Spitzenkorper features characteristic of particular fungi, and (b) dynamic pleomorphism-gradual or rapid changes in size, shape, and position of the Spitzenkorper within a single hyphal tip. Several components associated with the Spitzenkorper were identified: (a) vesicle cluster, (b) vesicle cloud, (c) differentiated core region(s) within the Spitzenkorper, (d) apical granules, (e) cytoplasmic filaments. Eight morphological patterns of Spitzenkorper organization are described in the higher fungi based on the shape and distribution of their components. An additional (ninth) pattern was recognized in the chytridiomycete Allomyces macrogynous from recent work by others. All these patterns appeared to be conserved at the genus level. In all patterns but one, a core region was observed by light microscopy. The Spitzenkorper not only exhibited spontaneous dynamic pleomorphism but also reacted to stress conditions (light, mechanical, and electrical fields). These reactions include migration of the Spitzenkorper back into the subapical zone and/or disassembly of its components. The understanding and conceptualization of this dynamic complex is problematic and should remain flexible enough to encompass the diversity of Spitzenkorper patterns and the dynamic pleomorphism of this specialized apical apparatus which appears to drive hyphal tip growth in the higher fungi.