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 th at is an example of species diverged through a founder event. These sp ecies exhibit asymmetric sexual isolation, courtships between D. plani tibia males and D. silvestris females being more successful than court ships between D. silvestris males and D. planitibia females. When anal yzing the signal-response courtship sequence in these species, we foun d that D. silvestris females responded to male circling by standing or preening while D. planitibia females required further signals from th e male to stop walking. The main reason for the reduced mating success rate of D. silvestris males with D. planitibia females was that the f emales responded to male circling by walking and the males did not pro ceed to the head-under-wings (HUW) position of a walking female. Anoth er critical phase in these courtships was the HUW position in D. silve stris, where males proceeded almost immediately to wing and leg vibrat ion. The courtships between D. planitibia male and D. silvestris femal e proceeded in a signal-response coordination until the male went to t he HUW position, where he fanned his wings for too long a period befor e proceeding to wing and leg vibration. Thus, it seems that the asymme tric isolation between D. planitibia (ancestral species) and D. silves tris (derived species) is mainly due to a loss of transitions in the s ignal-response chain of D. silvestris. A change in the behavior of the males in the HUW position has caused further isolation between the sp ecies in both directions.