Convergent evolution of sex-specific leg ornaments in Drosophilidae – from cells to structures

Sexually dimorphic morphological traits are among the fastest evolving animal features. Similar sex-specific structures have sometimes evolved independently in multiple lineages, presumably as targets of parallel sexual selection. In such cases, comparing the cellular mechanisms that generate these structures in different species can elucidate the interplay between selection and developmental constraint in evolution. In Drosophilidae, male-specific tarsal brushes on the front legs are found in at least four separate lineages. In this study, we combine phylogenetic reconstruction with developmental analyses and behavioral observations to investigate the evolutionary origin of these structures. We show that the sex brush has evolved independently at least three times from sexually monomorphic ancestral morphology. However, all sex brushes have very similar fine structure and develop through indistinguishable cellular processes, providing a striking example of developmental convergence. In all examined species, males use their sex brushes to grasp the female abdomen prior to copulation. We discuss potential reasons why convergent evolution of novel structures is rare even in the face of similar functional demands.