Rewired gene interactions during evolution of the analia and genitalia in <i>Drosophila</i>

Drosophila ventral appendages are considered to be serially homologous structures derived from a ventral appendage 'ground state' and shaped by different Hox inputs. The male and female terminalia (analia and genitalia) are highly derived ventral structures defined by Abdominal-B and the sex-determination pathway. In the legs and antennae, a combination of the transcription factors (TFs) C15, LIM1 homeobox 1 (Lim1) and Aristaless (Al) is required for tarsal claw and arista development, respectively. However, the roles of these TFs in terminalia development remained unexplored. We investigated the expression and function of C15, Lim1 and Al during male and female terminalia development. We found that C15 plays distinct roles in males and females, repressing male clasper bristles while promoting bristles on the female epiproct. Contrasting with antennae and legs, C15, Lim1 and Al are not all simultaneously co-expressed in any terminal structure in either sex, indicating interactions among these factors differ across these appendages. Nevertheless, we inferred interactions between C15 and other factors reflecting similarities between leg and male clasper development. Finally, we identified a male-specific C15 enhancer active in male claspers but not in the female epiproct, legs or antennae. This C15 enhancer modularity may underpin tissue- and sex-specific regulatory logic.