Microtubules coordinate cellularization in a coenocytic relative of animals

Cellularization is the coordinated division of a multinucleate cell into many cells, a common life cycle transition across eukaryotes. In the best-studied model of this process, Drosophila melanogaster , cellularization requires actin and microtubule networks, as well as membrane vesicle delivery to drive the synchronous invagination of plasma membrane furrows. To identify unifying principles, we investigate cellularization in the ichthyosporean Sphaeroforma arctica , a close relative of animals with a multinucleate life cycle stage. Using live-cell imaging, ultrastructure expansion microscopy, and volume electron microscopy, we define the membrane, microtubule, and actin dynamics that accompany cellularization. Using pharmacological inhibitors and centrifugation, we show that the microtubule network positions nuclei evenly at the cortex, thereby specifying furrow positioning. Disrupting microtubules perturbed nuclear position, furrow spacing, and the efficiency of furrow ingression, leading eventually to cells of variable size and nuclear content. Our findings indicate that the microtubule network sets the spatial framework within which actomyosin and membrane delivery drive cellularization and establish S. arctica as a tractable model for comparative studies of this eukaryotic life cycle strategy.