The receptor-like tyrosine phosphatase Lar is required for epithelial planar polarity and for axis determination within Drosophila ovarian follicles

The follicle cell monolayer that encircles each developing Drosophila oocyt e contributes actively to egg development and patterning, and also represen ts a model stem cell-derived epithelium. We have identified mutations in th e receptor-like transmembrane tyrosine phosphatase Lar that disorganize fol licle formation, block egg chamber elongation and disrupt Oskar localizatio n, which is an indicator of oocyte anterior-posterior polarity. Alterations in actin filament organization correlate with these defects. Actin filamen ts in the basal follicle cell domain normally become polarized during stage 6 around the anterior-posterior aids defined by the polar cells, but mutat ions in Lar frequently disrupt polar cell differentiation and actin polariz ation. Lar function is only needed in somatic cells, and (for Oskar localiz ation) its action is autonomous to posterior follicle cells. Polarity signa ls may be laid down by these cells within the extracellular matrix (ECM), p ossibly in the distribution of the candidate Lar ligand Laminin A, and read out at the time Oskar is localized in a Lar-dependent manner. Lar is not r equired autonomously to polarize somatic cell actin during stages 6. We sho w that Lar acts somatically early in oogenesis, during follicle formation, and postulate that it functions in germarium. intercyst cells that are requ ired for polar cell specification and differentiation. Our studies suggest that positional information can be stored transiently in the ECM. A major f unction of Lar may be to transduce such signals.