The establishment of the segmental pattern in the Drosophila embryo is directed by three sets of maternal genes: the anterior, the terminal and the posterior group of genes. Embryos derived from females mutant for one of the posterior group genes lack abdominal segmentation. This phenotype can be rescued by transplantation of posterior pole plasm into the abdominal region of mutant embryos. We transplanted posterior pole plasm into the middle of embryos mutant either for the posterior, the anterior and posterior, or all three maternal systems and monitored the segmentation pattern as well as the expression of the zygotic gap gene Krüppel in control and injected embryos. We conclude that polarity and identity of the abdominal segments do not depend on the relative concentration of posterior activity but rather on the position of gap gene expression. By changing the pattern of gap gene expression, the orientation of the abdomen can be reversed. These experiments suggest that maternal gene products act in a strictly hierarchical manner. The function of the maternal gene products becomes dispensable once the position of the zygotically expressed gap genes is determined. Subsequently the gap genes will control the pattern of the pair-rule and segment polarity genes.

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