Left-right asymmetries recurrently evolve in animals but the underlying developmental mechanisms are unknown. In most Drosophila species, the male genitalia is symmetric and undergoes a clockwise 360° rotation during development. In D. melanogaster, this tissue remodeling process is directed by class I myosins: in MyoID mutants the rotation is reversed and the genitalia remains symmetric. Males of Drosophila pachea have evolved unique left-right asymmetric genital organs and a characteristic right-sided copulation posture in the past 3-6 million years. To test if these asymmetries in morphology and behavior evolved via the recruitment of pre-existing directional cues controlled by MyoID, we used CRISPR to knockout MyoID in D. pachea. Strikingly, mutant males undergo a reverse genitalia rotation and develop mirror-image asymmetric genitalia, indicating that MyoID controls both the direction of genitalia rotation direction and morphological asymmetry. Although genital asymmetry is reversed, MyoID mutants still adopt a wild-type right-sided copulation posture. Our results show that MyoID and its asymmetry guidance role were recruited for the evolution of a novel left-right asymmetry in D. pachea and that lateralized copulation behavior is determined by other mechanisms. The evolution of a new left-right organ size asymmetry in D. pachea involved recruitment of existing directional cues.