DROSOPHILA OOCYTE LOCALIZATION IS MEDIATED BY DIFFERENTIAL CADHERIN-BASED ADHESION

In a Drosophila follicle the oocyte always occupies a posterior positi on among a group of sixteen germline cells. Although the importance of this cell arrangement for the subsequent formation of the anterior-po sterior axis of the embryo is well documented(1-4), the molecular mech anism responsible for the posterior localization of the oocyte was unk nown. Here we show that the hemophilic adhesion molecule DE-cadherin(5 -7) mediates oocyte positioning. During follicle biogenesis, DE-cadher in is expressed in germline (including oocyte) and surrounding follicl e cells, with the highest concentration of DE-cadherin being found at the interface between oocyte and posterior follicle cells. Mosaic anal ysis shows that DE-cadherin is required in both germline and follicle cells for correct oocyte localization, indicating that germline-soma i nteractions may be involved in this process. By analysing the behaviou r of the oocyte in follicles with a chimaeric follicular epithelium, w e find that the position of the oocyte is determined by the position o f DE-cadherin-expressing follicle cells, to which the oocyte attaches itself selectively. Among the DE-cadherin positive follicle cells, the oocyte preferentially contacts those cells that express higher levels of DE-cadherin. On the basis of these data, we propose that in wild-t ype follicles the oocyte competes successfully with its sister germlin e cells for contact to the posterior follicle cells, a sorting process driven by different concentrations of DE-cadherin. This is, to our kn owledge, the first in vivo example of a cell-sorting process that depe nds on differential adhesion mediated by a cadherin.