Divergence and correlated evolution of male wing spot and courtship display between <i>Drosophila nepalensis</i> and <i>D. trilutea</i>

Male-specific wing spots are usually associated with wing displays in the courtship behavior of Drosophila and may play important roles in sexual selection. Two closely related species, D. nepalensis and D. trilutea, differ in wing spots and scissoring behavior. Here we compare male morphological characters, pigmentation intensity of male wing spots, wing-scissoring behavior, courtship songs and reproductive isolation between two species. F1 fertile females and sterile males result from the cross between females of D. nepalensis and males of D. trilutea. The pigmentation of wing spots is significantly weaker in D. trilutea than in D. nepalensis and the F1 hybrid. Males scissor both wings in front of the female during courtship, with a posture spreading wings more widely, and at a faster frequency in D. nepalensis than in D. trilutea and the F1s. Males of D. trilutea vibrate wings to produce two types (A and B) of pulse songs, whereas D. nepalensis and the F1s sing only type B songs. The incidence of wing vibration and scissoring during courtship suggests that wing vibration is essential but scissoring is a facultative courtship element for successful mating in both species. The association between the darker wing spots with more elaborate scissoring might be the consequence of correlated evolution of these traits in D. nepalensis, however D. trilutea retains wing scissoring during courtship despite having weaker pigmentation of wing spots. The genetic architecture of two traits differs in the F1s, consistent with maternal or sex-linked effects for spots but non-additive effects for scissoring.