Cnidarian oocytes reveal conserved actin-driven mechanisms of female meiosis

Oocyte meiosis is a specialized form of cell division adapted to produce a single haploid egg for fertilization. Diverse actin-driven mechanisms have essential roles in supporting these highly asymmetric divisions of the large oocyte. Whether these represent exotic adaptations in individual species or whether actin has broadly conserved functions in animal oocytes remains unclear. To address this, we established live-imaging assays, combined with targeted perturbations and biophysical modeling, in the non-bilaterian jellyfish Clytia hemisphaerica . We show that in Clytia, a nuclear F-actin network stabilizes the large oocyte nucleus. Upon meiotic entry, a transient F-actin shell forms to facilitate nuclear envelope rupture, followed by chromosome congression driven by the collapse of the nuclear F-actin network and capture by microtubules. Finally, the forming spindle is transported to the cell periphery by cytoplasmic flows produced by a wave of cortical contraction. Together, the presence of these actin-driven mechanisms in a basal metazoan evidence their ancient origin, establishing a metazoan complement of conserved molecular modules required for oocyte divisions.