The Drosophila gene gurken participates in a signaling process that occurs between the germ line and the somatic cells (follicle cells) of the ovary. This process is required for correct patterning of the dorsoventral axis of both the egg and the embryo. gurken produces a spatially localized transcript which encodes a TGF-alpha-like molecule (Neuman-Silberberg and Schupbach, Cell 75, 165–174, 1993). Mutations in gurken cause a ventralized phenotype in egg and embryo. To determine whether the gurken gene product plays an instructive role in dorsoventral patterning, we constructed females containing extra copies of a gurken transgene. Such females produce dorsalized eggs and embryos, which is expected if gurken acts as a limiting factor in the dorsoventral patterning process. In addition, the expression pattern of the gene rhomboid in the follicle cells is altered in ovaries of females containing extra copies of gurken. Our results indicate that changing gurken dosage in otherwise wild-type ovaries is sufficient to alter the number of somatic follicle cells directed to the dorsal fate. Therefore the gurken-torpedo signaling process plays an instructive role in oogenesis. It induces dorsal cell fates in the follicle cell epithelium and it controls the production of maternal components that will direct the embryonic dorsoventral pattern after fertilization.

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