In vivo analysis of Drosophila bicoid mRNA localization reveals a novel microtubule-dependent axis specification pathway.

Drosophila bicoid mRNA is synthesized in the nurse cells and transported to the oocyte where microtubules and Exuperantia protein mediate localization to the anterior pole. Fluorescent bicoid mRNA injected into the oocyte displays nonpolar microtubule-dependent transport to the closest cortical surface, and the oocyte microtubule cytoskeleton lacks clear axial asymmetry. Nonetheless, bicoid mRNA injected into the nurse cell cytoplasm, withdrawn, and injected into a second oocyte shows microtubule-dependent transport to the anterior cortex. Nurse cells require microtubules and Exuperantia to support anterior transport of bicoid mRNA, and microtubules are required for bicoid mRNA-Exuperantia particle coassembly. We propose that microtubule-dependent Exuperantia-bicoid mRNA complex formation in the nurse cell cytoplasm allows anterior-specific transport on a grossly nonpolar oocyte microtubule network.