The nuclear-to-cytoplasmic ratio drives cellularization in the close animal relative Sphaeroforma arctica

The ratio of nuclear content to cytoplasmic volume (N/C ratio) is a key regulator driving the maternal-to -zy-gotic transition in most animal embryos. Altering this ratio often impacts zygotic genome activation and de-regulates the timing and outcome of embryogenesis.1-3 Despite being ubiquitous across animals, little is known about when the N/C ratio evolved to control multicellular development. Such capacity either origi-nated with the emergence of animal multicellularity or was co-opted from the mechanisms present in unicel-lular organisms.4 An effective strategy to tackle this question is to investigate the close relatives of animals exhibiting life cycles with transient multicellular stages.5 Among these are ichthyosporeans, a lineage of pro-tists undergoing coenocytic development followed by cellularization and cell release.6-8 During cellulariza-tion, a transient multicellular stage resembling animal epithelia is generated, offering a unique opportunity to examine whether the N/C ratio regulates multicellular development. Here, we use time-lapse microscopy to characterize how the N/C ratio affects the life cycle of the best-studied ichthyosporean model, Sphaero-forma arctica. We uncover that the last stages of cellularization coincide with a significant increase in the N/C ratio. Increasing the N/C ratio by reducing the coenocytic volume accelerates cellularization, whereas decreasing the N/C ratio by lowering the nuclear content halts it. Moreover, centrifugation and pharmacolog-ical inhibitor experiments suggest that the N/C ratio is locally sensed at the cortex and relies on phosphatase activity. Altogether, our results show that the N/C ratio drives cellularization in S. arctica, suggesting that its capacity to control multicellular development predates animal emergence.