Mutations Affecting Embryonic F-Actin Reorganization also Affect Separation of Nuclei from their Sisters and from the Cortex in Drosophila Cleavage Embryos: (Drosophila/nuclear migration, F-actin, mutants).

We showed in Drosophila that nuclear migration was reduced all through cleavage stages in embryos with any one of the maternal-effect mutations, gs(1)N441 and gs(1)N26, in which F-actin reorganization in cleavage embryos is disordered. Moreover, we determined nuclear positions in embryos at cycle 1 and 2 in the wild type and two mutants, gs(1)N441 and gs(1)N26, in order to test if the nuclear migration is regulated within a nuclear cycle. At cycle 1, there was no difference in nuclear position among the strains that we observed. At cycle 2 the two sister nuclei had already migrated posteriorly in the wild type. However, migration was not detectable at cycle 2 in the mutants. Besides, the two sister nuclei were less-separated from each other, and orientation of the two nuclei with regard to the anteroposterior axis was random, different from the wild type. These results support the hypothesis that F-actin is involved in the regulation to separate cleavage nuclei from each other and from the egg cortex. This regulation is apparently required for posteriorward nuclear migration, and for synchronous nuclear arrival in the whole egg cortex.