Dual-spindle formation in zygotes keeps parental genomes apart in early mammalian embryos.

At the beginning of mammalian life, the genetic material from each parent meets when the fertilized egg divides. It was previously thought that a single microtubule spindle is responsible for spatially combining the two genomes and then segregating them to create the two-cell embryo. We used light-sheet microscopy to show that two bipolar spindles form in the zygote and then independently congress the maternal and paternal genomes. These two spindles aligned their poles before anaphase but kept the parental genomes apart during the first cleavage. This spindle assembly mechanism provides a potential rationale for erroneous divisions into more than two blastomeric nuclei observed in mammalian zygotes and reveals the mechanism behind the observation that parental genomes occupy separate nuclear compartments in the two-cell embryo.