The Drosophila nucleoporin ELYS is required for parental chromosome arrangement at fertilization.

One key aspect of fertilization is the unification of the maternal and paternal genomes driven by the first mitotic spindle. However, little is known about the mechanisms that underlie the formation of a bipolar spindle that interacts with the two discrete chromosome sets in juxtaposition. We here show that, in Drosophila, the maternally provided ELYS-an evolutionarily conserved subunit of the nuclear pore complex-localizes to female and male pronuclei and then redistributes to the interior of the spindle and the resulting zygotic nuclei. Both Elys loss-of-function mutations and ELYS overexpression in the female germline were associated with maternal-effect lethality. Our cytological studies of fertilized eggs revealed that ELYS is primarily involved in the apposition of female and male pronuclei, potentially impacting the parental genome configuration of the first mitotic spindle. We propose that pronuclear apposition is essential for centrosome localization at the emergent pronuclear junction to promote bipolar spindle formation for the first mitosis. In addition, we discuss the possible involvement of ELYS in interspecific hybrid incompatibility.