In Drosophila melanogaster oocytes, the nucleus moves from a posterior localization towards an anterior corner, which then becomes the dorsal axis. This movement was thought to be powered by the dynein motor and microtubules that are simultaneously nucleated along the anterior lateral cortex, setting up the antero-posterior axis. But Daniel St. Johnston and colleagues have now found that the nucleus is pushed away by the force provided by microtubules polymerizing from organizing centres localized behind the nucleus at the posterior, thus dissociating migration and dorso-ventral determination from antero-posterior axis set-up (Science http://doi.org/hwj; 2012).

Using live imaging, they noticed a deformation in the nucleus in the direction of movement, indicating the existence of a pushing force. They observed an enrichment of the fluorescently tagged plus-end-associated protein EB1–GFP near the indentation, and that depolymerization of microtubules decreased both the deformation and this enrichment. Further, centrosomal proteins were localized behind the nucleus, indicating that EB1–GFP-labelled microtubules grew from this location. Laser-mediated ablation of the centrosomes prevented the deformation, and ectopic anterior deformations were seen in mutants with mislocalized centrosomes. The authors calculated that the force exerted by only a few microtubules would be sufficient to drag the nucleus away from the posterior, consistent with their estimation of the number of microtubules touching the nucleus. As indentations were seen on nuclei still anchored at the posterior, the adjacent follicular cell may maintain the nuclei in place until a release signal is sent. Discovering the nature of this signal is the next challenge.