Growing Microtubules Push the Oocyte Nucleus to Polarize the Drosophila Dorsal-Ventral Axis

Pull Me-No!-Push You The Drosophila dorsal-ventral (DV) axis is polarized by the movement of the nucleus from the posterior end of the oocyte to its anterior margin. It has long been assumed that the nucleus is pulled to the anterior end by the molecular motor dynein along the polarized microtubule cytoskeleton that defines the anterior-posterior (AP) axis. Using live imaging, Zhao et al. (p. 999, published online 12 April; see the Perspective by Bowerman and O'Rourke) now demonstrate that the nucleus is pushed toward the anterior by the force exerted by growing microtubules hitting its posterior side. DV polarity thus depends on the posterior positioning of the microtubule organizing center rather than on AP axis formation. The addition of tubulin monomers to microtubules provides the force to relocate the oocyte nucleus. The Drosophila dorsal-ventral (DV) axis is polarized when the oocyte nucleus migrates from the posterior to the anterior margin of the oocyte. Prior work suggested that dynein pulls the nucleus to the anterior side along a polarized microtubule cytoskeleton, but this mechanism has not been tested. By imaging live oocytes, we find that the nucleus migrates with a posterior indentation that correlates with its direction of movement. Furthermore, both nuclear movement and the indentation depend on microtubule polymerization from centrosomes behind the nucleus. Thus, the nucleus is not pulled to the anterior but is pushed by the force exerted by growing microtubules. Nuclear migration and DV axis formation therefore depend on centrosome positioning early in oogenesis and are independent of anterior-posterior axis formation.

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