Three-dimensional cellularization in chytrid fungi uses distinct mechanisms from those driving one- and two-dimensional cytokinesis in animals and yeast

Chytrid fungi offer a unique opportunity to study three-dimensional cellularization, where nuclei that start out homogeneously distributed throughout the cytoplasm are synchronously compartmentalized into daughter cells. This process addresses geometric challenges not encountered in conventional cytokinesis, where nuclei are organized in a one-dimensional line, or in Drosophila cellularization, which is organized in a two-dimensional plane—both of which allow nuclei direct access to cytokinetic cues placed at the plasma membrane. Here, we show that three-dimensional cellularization in the chytrid Spizellomyces punctatus proceeds through a stepwise program, beginning with the transient migration of nuclei and their associated centrosomes to the plasma membrane. At this point, centrosome-associated vesicles form and initiate membrane invagination, establishing tubular furrows that expand and branch, ultimately creating a foam-like tessellation of tightly packed polyhedral compartments, each containing a single nucleus and bearing a cilium. Using a combination of small molecule inhibitors and laser ablation, we demonstrate that, similar to cytokinesis and Drosophila cellularization, chytrid cellularization is powered by actomyosin networks. We also show that while microtubules are important for patterning, they appear dispensable for cellularization. Finally, we suggest that chytrids may have incorporated mechanisms associated with ciliogenesis in animals to coordinate the association of internal nuclei with actomyosin networks and membranes. These data highlight the value of using chytrid fungi to study the unique geometric problems associated with three-dimensional cellularization.