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 maternal-to-zygotic transition in most animal embryos. Altering this ratio often impacts zygotic genome activation and deregulates 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 originated with the emergence of animal multicellularity or was co-opted from mechanisms present in unicellular organisms [4]. An effective strategy to tackle this question is to investigate close relatives of animals exhibiting life cycles with transient multicellular stages [5]. Among these are ichthyosporeans, a lineage of protists undergoing coenocytic development followed by cellularization and cell release [6-8]. During cellularization, 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, Sphaeroforma 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 while decreasing the N/C ratio by lowering the nuclear content halts it. Moreover, centrifugation and pharmacological 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.