INTRODUCTION Anterior/posterior (A/P) and dorsal/ventral (D/V) polarity in the Drosophila oocyte is established through a series of inductive interactions between the oocyte and the adjacent monolayer of somatic follicle cells (reviewed in Anderson, 1995; Grünert and St. Johnston, 1996; Ray and Schüpbach, 1996; Rongo and Lehmann, 1996). The oocyte, which is located at the posterior of a cluster of germline-derived nurse cells, induces the adjacent follicle cells to adopt a posterior fate, rather than a default anterior fate (González-Reyes and St. Johnston, 1994). This induction requires the activity of gurken (grk) in the germline and torpedo (top) in the follicle cells (González-Reyes et al., 1995; Roth et al., 1995), implicating the Egf receptor (Egfr) signalling pathway in the establishment of posterior cell fates. Grk is a transforming growth factor (TGF)-α-like molecule with an epidermal growth factor (EGF) domain (Neuman-Silberberg and Schüpbach, 1993), while Top is the homologue of the Egfr (Livneh et al., 1985; Price et al., 1989; Wadsworth et al., 1985). Egfr signalling activates genes in the posterior follicle cells, including pointed (Morimoto et al., 1996). Following Egfr signalling, posterior follicle cells signal to the underlying oocyte to establish A/P polarity. This second signal requires the function of Notch (N) and Delta (Dl) in the follicle cells, and mago nashi and the catalytic subunit of protein kinase A in the germline (Ruohola et al., 1991; Larkin et al., 1996; Micklem et al., 1997; Newmark et al., 1997; Lane and Kalderon, 1994). Perturbation of this ill-defined somatic signal leads to the mislocalization of morphogenetic determi- nants along the A/P axis. bicoid (bcd) mRNA, which is normally localized to the anterior pole, becomes mislocalized to both ends of the oocyte and posterior components are mis- localized to the centre (Ruohola et al., 1991; González-Reyes and St. Johnston, 1994; Lane and Kalderon, 1994; González- Reyes et al., 1995; Roth et al., 1995; Larkin et al., 1996). Current models propose that the somatic signal acts to re- organize the oocyte cytoskeleton (Lane and Kalderon, 1994; Ruohola et al., 1994). When it is perturbed, microtubules grow from both poles of the oocyte, creating a bipolar array with the growing ends of microtubules in the centre of the oocyte. Formation of the D/V axis requires additional inductive events between the germline and follicle cells (reviewed in Grünert and St. Johnston, 1996; Ray and Schüpbach, 1996; Rongo and Lehmann, 1996). Following A/P axis specification, grk mRNA accumulates between the nucleus and the cell membrane at the future dorsal-anterior margin of the oocyte (Neuman-Silberberg and Schüpbach, 1993). Spatially restricted Grk is believed to activate Top in the adjacent epi- thelium, thereby specifying dorsal follicle cell fate and restrict- 4661 Development 124, 4661-4671 (1997) Printed in Great Britain © The Company of Biologists Limited 1997 DEV8465 We describe a mutant, maelstrom, that disrupts a previ- ously unobserved step in mRNA localization within the early oocyte, distinct from nurse-cell-to-oocyte RNA transport. Mutations in maelstrom disturb the localization of mRNAs for Gurken (a ligand for the Drosophila Egf receptor), Oskar and Bicoid at the posterior of the devel- oping (stage 3-6) oocyte. maelstrom mutants display phe- notypes detected in gurken loss-of-function mutants: posterior follicle cells with anterior cell fates, bicoid mRNA localization at both poles of the stage 8 oocyte and ven- tralization of the eggshell. These data are consistent with the suggestion that early posterior localization of gurken mRNA is essential for activation of the Egf receptor pathway in posterior follicle cells. Posterior 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 distribution in the cytoplasm of the nurse cells and the oocyte until the protein disappears in stage 7 of oogenesis. Key words: Drosophila, axis formation, RNA localization, Egfr signalling, asymmetry, maelstrom, gurken SUMMARY maelstrom is required for an early step in the establishment of Drosophila oocyte polarity: posterior localization of grk mRNA Nigel J. Clegg 1 , Deanna M. Frost 1, *, Michele Keller Larkin 1 , Lakshman Subrahmanyan 1,2 , Zev Bryant 1 and Hannele Ruohola-Baker 1,† 1 Department of Biochemistry, J581 Health Science Building, Box 357350, University of Washington, Seattle, WA 98195-7350, USA 2 Center for Bioengineering, University of Washington, Seattle WA 98195, USA *Present address: Virginia Merrill Bloedel Hearing Research Center, University of Washington, Box 357923, Seattle WA, 98195-7923, USA † Author for correspondence (e-mail: hannele@u.washington.edu)