A 2012–13 survey on Penang Island, Malaysia, revealed the existence of both Drosophila ananassae and Drosophila parapallidosa, the latter of which carries chromosomes Y and 4 from D. ananassae and thus is of hybrid origin. We collected the flies again from the same location in 2018. The hybrid population remained present, which suggests that the D. parapallidosa of hybrid origin does not represent a mere transient population but is stable. Why do these two species coexist irrespective of gene flow? We realized that body size is generally larger in D. ananassae than in D. parapallidosa, which constitutes a new character with which to discriminate these species; previously the number of sex comb teeth was the only diagnostic trait. Character displacement was not detected, however, for those traits. We crossed these two species, which resulted in offspring that had an altered genomic constitution. The body size of D. ananassae was dominant, and the presence of chromosomes Y and 4 did not have a significant effect on body size. By contrast, the presence of chromosome 4 from D. ananassae significantly affected the number of sex comb teeth. Even flies having a genomic constitution similar to that of the Penang D. parapallidosa exhibited a number of sex comb teeth that was intermediate between the two species. We propose that the D. parapallidosa sex comb character underwent selection during evolution of the Penang Island population. Reproductive interference between the species, presumably caused by signal jamming, was detected.
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