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Abstract

A small set of maternal genes controls the basic longitudinal pattern of the Drosophila embryo. For the posterior pattern, five genes are necessary both for abdominal segmentation and for pole cell formation1–3. Cytoplasmic transplantations involving oskar (osk) mutant embryos have suggested that the pole plasm serves as a source of a signal required in the more anterior abdominal region3. Here we present evidence that the maternal gene pumilio (pum) is involved in the transport of this signal to the abdominal region. In pum embryos (that is, embryos from females of genotype pum /pum) only the abdomen is affected, whereas the pole plasm seems normal. Transplantation experiments reveal that pum pole plasm contains the abdominal signal but that it cannot reach the target site, the abdomen. Abdominal segmentation is restored when the physical separation between pole plasm and abdominal region is overcome either by transplantation of pum pole plasm into the abdominal region of pum embryos or by genetic means in double mutants with torsolike (in mutant torsolike embryos the abdominal region is juxtaposed to the pole plasm).