Mosaic analysis in the Drosophila ovary reveals a common hedgehog-inducible precursor stage for stalk and polar cells

The fates of two small subgroups of the ovarian follicle cells appear to be linked: mutations in Notch, Delta, fs(1)Yb, or hedgehog cause simultaneous defects in the specification of stalk cells and polar cells. Both of these subgroups are determined in the germarium, and both cease division early i n oogenesis. To test the possibility that these subgroups are related by li neage, we generated dominantly marked mitotic clones in ovaries. Small, res tricted clones in stalk cells and polar cells were found adjacent to each o ther at a frequency much too high to be explained by independent induction. We therefore propose a model in which stalk cells and polar cells are deri ved from a precursor population that is distinct from the precursors for ot her follicle cells. We support and extend this model by characterization of mutants that affect stalk and polar cell formation. We find that ectopic e xpression of Hedgehog can induce both polar and stalk cell fate, presumably by acting on the precursor stage. In contrast, we find that stall affects neither the induction of the precursors nor the decision between the stalk cell and polar cell fate but, rather, some later differentiation step of st alk cells. In addition, we show that ectopic polar and stalk cells disturb the anterior-posterior polarity of the underlying oocyte.