Animals have body parts made of similar cell types located at different axial positions, such as limbs. The identity and distinct morphology of each structure is often specified by the activity of different "master regulator" transcription factors. Although similarities in gene expression have been observed between body parts made of similar cell types, how regulatory information in the genome is differentially utilized to create morphologically diverse structures in development is not known. Here, we use genome-wide open chromatin profiling to show that among the Drosophila appendages, the same DNA regulatory modules are accessible throughout the genome at a given stage of development, except at the loci encoding the master regulators themselves. In addition, open chromatin profiles change over developmental time, and these changes are coordinated between different appendages. We propose that master regulators create morphologically distinct structures by differentially influencing the function of the same set of DNA regulatory modules.
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