Mitosis is a highly regulated process that facilitates faithful segregation of genetic material into daughter cells. During mitosis in animal cells, nuclear envelope breakdown (NEBD) allows microtubules emanating from cytoplasmic centrosomes to connect to the duplicated chromosomes. The nuclear envelope (NE) reforms at the end of mitosis, generating a single nucleus in each daughter cell. NEBD following fertilization in a 1-cell embryo is poorly understood, especially in mammals where visualizing this process is challenging. It is assumed that once the maternal and paternal nuclei are in close apposition NEBD is initiated. The signals that promote timely NEBD are unknown. To visualize mitotic events following fertilization we utilized early-stage embryos from Caenorhabditis elegans. Polo-like kinase 1 (Plk1) is a conserved kinase involved in multiple steps of mitosis. While Plk1 function is studied extensively in somatic cells, little is known about its function during embryonic development. We observed that in C. elegans embryo, growth of a temperature sensitive plk-1 mutant (plk-1ts) at the semi-permissive temperature led to the formation of two nuclei per cell, containing either the maternal or paternal DNA. This paired-nuclei phenotype was caused by a defect in NEBD, and specifically the formation of a gap in the NE at the interface between the maternal and paternal pronuclei during the first mitosis. In the plk-1ts mutant we also observed a defect in chromosome alignment of the maternal and paternal metaphase plates relative to each other. Since PLK-1 is known to associate with chromosomes, the NEBD defect in a plk-1ts mutant could be due to a direct requirement for PLK-1 at the NE region where the membrane gap is formed. We hypothesize that chromosome alignment may be necessary to achieve a critical concentration of chromosome-bound PLK-1 at the site of the future membrane gap. Alternatively, the NEBD in the plk-1ts mutant could be due to an indirect requirement for PLK-1 in chromosome alignment, which in turn leads to a signal of an unknown nature that is necessary for gap formation in NE. Either way, PLK-1- dependent gap formation in the NE is essential for parental chromosome mixing. To further understand this process, we utilized data from a RNAi screen where we down regulated ~2000 genes critical for embryogenesis, and recorded changes in nuclear morphology in embryonic cells. To date, we have identified 17 candidates that, when down regulated result in paired nuclei formation. We are now investigating how inactivation of these gene functions leads to paired nuclei formation, and whether there’s any functional relationship between these genes and the PLK-1 mediated NEBD regulation pathway.