Change in the signal-response sequence responsible for asymmetric isolation between Drosophila planitibia and Drosophila silvestris.

Drosophila planitibia and Drosophila silvestris form a species pair that is an example of species diverged through a founder event. These species exhibit asymmetric sexual isolation, courtships between D. planitibia males and D. silvestris females being more successful than courtships between D. silvestris males and D. planitibia females. When analyzing the signal-response courtship sequence in these species, we found that D. silvestris females responded to male circling by standing or preening while D. planitibia females required further signals from the male to stop walking. The main reason for the reduced mating success rate of D. silvestris males with D. planitibia females was that the females responded to male circling by walking and the males did not proceed to the head-under-wings (HUW) position of a walking female. Another critical phase in these courtships was the HUW position in D. silvestris, where males proceeded almost immediately to wing and leg vibration. The courtships between D. planitibia male and D. silvestris female proceeded in a signal-response coordination until the male went to the HUW position, where he fanned his wings for too long a period before proceeding to wing and leg vibration. Thus, it seems that the asymmetric isolation between D. planitibia (ancestral species) and D. silvestris (derived species) is mainly due to a loss of transitions in the signal-response chain of D. silvestris. A change in the behavior of the males in the HUW position has caused further isolation between the species in both directions.