The evolutionary origins of complex morphological structures such as the vertebrate eye or insect wing remain one of the greatest mysteries of biology. Recent comparative studies of gene expression imply that new structures are not built from scratch, but rather form by co-opting preexisting gene networks. A key prediction of this model is that upstream factors within the network will activate their preexisting targets (i.e., enhancers) to form novel anatomies. Here, we show how a recently derived morphological novelty present in the genitalia of D. melanogaster employs an ancestral Hox-regulated network deployed in the embryo to generate the larval posterior spiracle. We demonstrate how transcriptional enhancers and constituent transcription factor binding sites are used in both ancestral and novel contexts. These results illustrate network co-option at the level of individual connections between regulatory genes and highlight how morphological novelty may originate through the co-option of networks controlling seemingly unrelated structures.
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