THE MAGO NASHI GENE IS REQUIRED FOR THE POLARIZATION OF THE OOCYTE AND THE FORMATION OF PERPENDICULAR AXES IN DROSOPHILA

Background: Drosophila axis formation requires a series of inductive i nteractions between the oocyte and the somatic follicle cells. Early i n oogenesis, Gurken protein, a member of the transforming growth facto r a. family, is produced by the oocyte to induce the adjacent follicle cells to adopt a posterior cell fate, These cells subsequently send a n unidentified signal back to the oocyte to induce the formation of a polarised microtubule array that defines the anterior-posterior axis. The polarised microtubules also direct the movement of the nucleus and gurken mRNA from the posterior to the anterior of the oocyte, where G urken signals a second time to induce the dorsal follicle cells, there by polarising the dorsal-ventral axis. Results: In addition to its pre viously described role in the localisation of oskar mRNA, the mago nas hi gene is required in tile germ line for the transduction of the pola rising signal from the posterior follicle cells. Using a new in vivo m arker for microtubules, we show that mago nashi mutant oocytes develop a symmetric microtubule cytoskeleton that leads to the transient loca lisation of bicoid mRNA to both poles, Furthermore, the oocyte nucleus often fails to migrate to the anterior, causing the second Gurken sig nal to be sent in the same direction as the first, This results in a n ovel phenotype in which the anterior of the egg is ventralised and the posterior dorsalised, demonstrating that the migration of the oocyte nucleus determines the relative orientation of the two principal axes of Drosophila. The mago nashi gene is highly conserved from plants to animals, and encodes a protein that is predominantly localised to nucl ei. Conclusions: The mago nashi gene plays two essential roles in Dros ophila axis formation: it is required downstream of the signal from th e posterior follicle cells for the polarisation of the oocyte microtub ule cytoskeleton, and has a second, independent role in the localisati on of oskar mRNA to the posterior of the oocyte.