Incipient sexual isolation in the nasuta-albomicans complex of Drosophila:No-choice experiments

Drosophila nasuta nasuta and Drosophila nasuta albomicans are cross-fertile races of Drosophila. Hybridization between these races in the laboratory h as given rise to new races (Cytoraces), among which karyotypic composition differs from one another and also from those of the parental races. In this study, we search for the evidence of incipient reproductive isolation amon g the parental races and four Cytoraces by assessing the fraction of no-mat ings, mating latency and copulation duration in all possible types of homo- and heterogamic crosses (N = 4184). In no-choice conditions, the latency t ime (time to initiation of copulation) is lower in homogamic crosses than i n heterogamic crosses for both parental races and Cytoraceu. Latency time a nd copulation duration are negatively correlated, whereas fraction of no ma tings is positively correlated with latency time. Thus these six closely re lated races of the nasuta-albomicans complex show the initiation of the ear liest stages of pre-zygotic isolation, manifested as a tendency for matings to be initiated earlier and more often, and for a longer duration, among h omogamic rather than heterogamic individuals.