THE DROSOPHILA AP AXIS IS POLARIZED BY THE CADHERIN-MEDIATED POSITIONING OF THE OOCYTE

The anterior-posterior axis of Drosophila originates from two symmetry -breaking steps during early oogenesis. First, one of the two pro-oocy tes within the cyst of 16 germline cells is selected to become the ooc yte, This cell then comes to lie posterior to the other germline cells of the cyst, thereby defining the polarity of the axis. Here we show that the oocyte reaches the posterior of the cyst in two steps. (1) Th e cyst flattens as it enters region 2b of the germarium to place the t wo pro-oocytes in the centre of the cyst, where they contact the poste rior follicle cells. (2) One cell is selected to become the oocyte and protrudes into the posterior follicle cell layer when the cyst rounds up on entering region 3. During this germ cell rearrangement, the com ponents of the hemophilic cadherin adhesion complex, DE-cadherin, Arma dillo and alpha-catenin, accumulate along the border between the oocyt e and the posterior follicle cells. Furthermore, the positioning of th e oocyte requires cadherin-dependent adhesion between these two cell t ypes, since the oocyte is frequently misplaced when DE-cadherin is rem oved from either the germline or the posterior follicle cells. We conc lude that the oocyte reaches the posterior of the germline cyst becaus e it adheres more strongly to the posterior follicle cells than its ne ighbours during the germ cell rearrangement that occurs as the cyst mo ves into region 3. The Drosophila anterior-posterior axis therefore be comes polarised by an unusual cadherin-mediated adhesion between a ger m cell and mesodermal follicle cells.