Microtubules sustain the fidelity of cellularization in a coenocytic relative of animals

Cellularization is the coordinated division of a multinucleate cytoplasm into many cells. ^1–3 Multinucleation is a common life cycle strategy observed across eukaryotic lineages, including in microbial eukaryotes, fungi, plants and animals, and is associated with the ability to transition to a unicellular state through cellularization. ⁴ In the best-studied model for this process, Drosophila melanogaster , cellularization requires the coordinated action of actin and microtubule (MT) networks to bring about the synchronous invagination of plasma membrane furrows, but the extent of conservation of these mechanisms across eukaryotes remains unknown. ^1,5,6 Here we investigate cellularization in the ichthyosporean Sphaeroforma arctica , a close relative of animals with a multinucleate life cycle stage. ^7–9 Using live cell imaging, ultrastructure expansion microscopy (U-ExM) and volume electron microscopy, we define the membrane, MT and actin dynamics that accompany cellularization in S. arctica . Using pharmacological inhibitors and centrifugation, we show that MTs, in addition to positioning nuclei, play a role in guiding nascent furrows to sustain equi-partitioning of nuclei and cytoplasm between daughter cells. Our findings indicate that cellularization is regulated through crosstalk between actin and MT networks, exhibiting mechanistic parallels with canonical cytokinesis, and establish S. arctica as a valuable model for investigating general principles of cellularization.