OOCYTE DETERMINATION AND THE ORIGIN OF POLARITY IN DROSOPHILA - THE ROLE OF THE SPINDLE GENES

The two main body axes in Drosophila become polarised as a result of a series of symmetry-breaking steps during oogenesis. Two of the sixtee n germline cells in each egg chamber develop as pro-oocytes, and the f irst asymmetry arises when one of these cells is selected to become th e oocyte. Anterior-posterior polarity originates when the oocyte then comes to lie posterior to the nurse cells and signals through the Gurk en/Egfr pathway to induce the adjacent follicle cells to adopt a poste rior fate. This directs the movement of the germinal vesicle and assoc iated gurken mRNA from the posterior to an anterior corner of the oocy te, where Gurken protein signals for a second time to induce the dorsa l follicle cells, thereby polarising the dorsal-ventral axis. Here me describe a group of five genes, the spindle loci, which are required f or each of these polarising events, spindle mutants inhibit the induct ion of both the posterior and dorsal follicle cells by disrupting the localisation and translation of gurken mRNA. Moreover, the oocyte ofte n fails to reach the posterior of mutant egg chambers and differentiat es abnormally. Finally, double mutants cause both pro-oocytes to devel op as oocytes, by delaying the choice between these two cells. Thus, t hese mutants reveal a novel link between oocyte selection, oocyte posi tioning and axis formation in Drosophila, leading us to propose that t he spindle genes act in a process that is common to several of these e vents.