(63) Pronuclear Junction Formation is Essential for Building a Functional Bipolar Spindle during the First Mitotic Division of Drosophila Embryos

Kazuyuki Hirai¹; Kyoichi Sawamura²

1. Department of Biology, Kyorin University School of Medicine, Mitaka, Japan

2. Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan

Abstract Text: Fertilization restores the diploid state by the unification of parental chromosomes. The developmental stage-specific mitotic spindle directs the process, but the assembly mechanisms remain unclear. We show novel findings that, in Drosophila fertilized eggs, the ELYS protein—an evolutionarily conserved subunit of the nuclear pore complex—localizes to the female and male pronuclei and is essential for their juxtaposition. In particular, ELYS abundance is critical for pronuclear spacing adjustment. The phenotypes shown by embryos in which the ELYS level was decreased or increased experimentally indicate that the abundance of maternally derived ELYS must fall within a certain range to ensure pronuclear apposition. We suggest that centrosome positioning at the emergent pronuclear junction is a basis for forming the first mitotic spindle around the two discrete parental chromosome groups. Furthermore, our functional and evolutionary test revealed that ELYS is involved in reproductive isolation between closely related Drosophila species. The cross-species ELYS exhibited a gain-of-function activity, causing the same fertilization failure induced by ELYS overproduction. Owing to multiple similarities in the configuration and function of the first mitotic spindle in the zygote of various animal species, our results may have important implications for elucidating the mechanisms that govern the first mitosis of the human zygote.