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Abstract

Segmentation of the Drosophila embryo involves expression of several classes of zygotic genes in response to positional information of maternal origin1. Zygotic expression of the gap genes is thought to be required for the subdivision of the embryo into several units of adjacent segments; pair-rule genes establish a repeat unit length of two segments and segment polarity genes specify parts of the single segment2–5. The positional information provided by the three classes of segmentation genes is interpreted for the establishment of segment identity using the homoeotic genes6–10. Molecular probes for several segmentation and homoeotic genes reveal a temporal and spatial pattern of gene expression in the young embryo11–18. Changes in the pattern of expression of these genes in mutant embryos provide evidence for a network of interactions among homoeotic and pair-rule genes19–23 that itself depends on the activity of gap genes24–27. We show here that the spatial limit of gap gene expression depends on bilateral interactions among gap genes.