Schematic of the mechanism by which zygotes regulate the localization of paternal chromosomes. RanGTP signaling and F-actin dynamics play central roles in spatially separating paternal chromosomes from maternal chromosomes in zygotes. On the plasma membrane before fertilization, RanGTP regulates Juno/CD9 structures and blocks egg–sperm fusion in proximity to maternal chromosomes. In the cytoplasm, following egg–sperm fusion, F-actin keeps paternal chromosomes away from maternal chromosomes.