MAELSTROM IS REQUIRED FOR AN EARLY STEP IN THE ESTABLISHMENT OF DROSOPHILA OOCYTE POLARITY - POSTERIOR LOCALIZATION OF GRK MESSENGER-RNA

We describe a mutant, maelstrom, that disrupts a previously unobserved step in mRNA localization within the early oocyte, distinct from nurs e-cell-to-oocyte RNA transport, Mutations in maelstrom disturb the loc alization of mRNAs for Gurken (a ligand for the Drosophila Egf recepto r), Oskar and Bicoid at the posterior of the developing (stage 3-6) oo cyte. maelstrom mutants display phenotypes detected in gurken loss-of- function mutants: posterior follicle cells with anterior cell fates, b icoid mRNA localization at both poles of the stage 8 oocyte and ventra lization of the eggshell. These data are consistent with the suggestio n that early posterior localization of gurken mRNA is essential for ac tivation of the Egf receptor pathway in posterior follicle cells. Post erior localization of mRNA in stage 3-6 oocytes could therefore be one of the earliest known steps in the establishment of oocyte polarity. The maelstrom gene encodes a novel protein that has a punctate distrib ution in the cytoplasm of the nurse cells and the oocyte until the pro tein disappears in stage 7 of oogenesis.