The Relationship Between Wing Spot And Wing Display In Drosophila Nepalensis And Its Closely Related Species D. Trilutea

Drosophila nepalensis and its closely related species D.trilutea are in the takahashiispecies-subgroup.Morphological characters and male genitalia are almost the same between these two species except the wing-spots.There is a black spot on the tip of the male wings of D.nepalensis,but nothing on D.trilutea.In order to understand what is the status of speciation between them,especially what is the role of the wing-spot in the sexual selection,in present study,videos regarding the courtship behavior were recorded and courtship elements were identified.Types and parameters of courtship song were analyzed.The fertility,wing spot size and sexual ratio of F1 and BC1 progenies were investigated based on the results of the cross and backcross experiments.Furthermore,the mate-choice tests were conducted to indicated which sex discriminates mates during the courtship and copulation.The following results were obtained from present study.1)Courtship peak,Cross and backcross experiments.The courtship behavior of D.nepalensis and D.trilutea was observed from the 2 days-old virgin flies.The courtship reached a peak when they were 6 days-old.Results of cross and backcross indicated that females of D.nepalensis and males of D.trilutea produced fertile female F1 progenies and sterile F1 males.The wing spot size of F1 males is significantly smaller than males of mother species.The wing spot size of backcross BC1 offsprings got smaller than males of F1.The combination between females of D.trilutea and males of D.nepalensis produced no offsprings,there was strong premating isolation in this combination.2)Sexual selection,and the influence of wing spots in the courtship.Results of mate-choice tests and wing spots modifying revealed that females distinguished mates in the courtship and during copulation.The copulation rate of D.trilutea decreased when artificial wing spots were added on the wings of male.Such results suggested that the strong sexual isolation in the cross combination of females of D.trilutea and males of D.nepalensis might due to the negative involvement of wing spots from D.nepalensis.3)Courtship behavior.The courtship procedure is similar in two species with behavioral elements of orientation,tapping,following,wing vibration,circling,wings display with scissoring and body swing,as well as attempt to mount.There is obvious difference on the wing display between two species.Males of D.nepalensis with black spots displayed small range of wing scissoring,rising wings from bigger 90 degree to 180 degree.Males of D.trilutea without spots waved wings with big range from smaller 90 degree to 180 degree.These small and big ranges of movement resulted in the fast and slow wing display for D.nepalensis and D.trilutea.In other words,D.nepalensis got more chances to show females his wing spots by small range of wing display.4)Types and parameters of courtship song.There are four types of pulse songs in D.trilutea,two of them are continue pulses with different pulse length.Others are individual pulses with difference cycle number.Whereas,males of D.nepalensis produced two types of pulse songs,one is the long length continue pulse song and the other is individual pulse with more cycle numbers.5)Wing spots along with special fast wing display in the courtship of D.nepalensis is a very interesting coevolution consequence between morphological character and courtship element.By the fast wing display,wing spots of male could be shown to the female.The wing display is not a necessary courtship element for successful copulation in D.nepalensis,even male flies have better strategies to show their wing spots than the D.trilutea does,it might be an alternate courtship element next to the courtship songs.In case of male competition for mates,wing display might be a risky behavior,because the harvest of courtship by wing display might be easily stolen by other males standing nearby to mount.Wing spots might give D.nepalensis the advantage in intraspecific or interspecific competition,thus obtaining the right to mate.In the D.trilutea,losing wing spots companied with the fewer males showing slower big range of wings display in the courtship.Such changed behavior might undergo continue loss in the future evolutionary progress.In conclusion,the results obtained from present study provide new data for understanding the mechanism of speciation among closely related species,especially the role of wing spots and wing display courtship element in the sexual selection.It means a lot in the research field of speciation and evolutionary genetics.