A Notch signal required for a morphological novelty in <i>Drosophila</i> has antecedent functions in genital disc eversion

The origin of morphological novelties has long fascinated biologists. Signaling pathways play important roles in the formation of novelties; however, the history of how they become integrated into new developmental programs remains unclear. Here, we investigated the evolution of the posterior lobe, a novel structure in the male genitalia of <i>Drosophila melanogaster</i>. We demonstrated that a Notch signaling center is required for the formation of this novelty and identified enhancers of the ligand <i>Delta</i>, which allowed us to track the evolutionary history of this signaling center. Unexpectedly, we found that the posterior lobe signaling center emerged from a preexisting role in genital disc eversion. We provide a likely mechanism by which Delta contributes to genital eversion through a network of apical extracellular matrix, which also became integrated into the posterior lobe program. This work demonstrates that novelties may be formed in the context of already complex developmental processes, by appending new roles to preexisting signals.