BEHAVIORAL REPRODUCTIVE ISOLATION IN DROSOPHILA-SILVESTRIS, D-HETERONEURA, AND THEIR F1 HYBRIDS (DIPTERA, DROSOPHILIDAE)

We investigated the role that courtship and aggressive interactions may have for the maintenance of reproductive isolation between Drosophila silvestris and D. heteroneura. We examined the behavioral bases of reproductive isolation between the parental species and we examined the courtship success of each sex of both reciprocal F-1 hybrids when paired with the parental species. We found reduced copulation success among heterotypic parental pairs compared to homotypic pairs, which was primarily due to the lack of courtship initiation between D. silvestris males and D. heteroneura females. When hybrid males from both reciprocal crosses were paired with parental females their copulation successes were not significantly different from that of parental males. In contrast, hybrid females from both crosses had reduced copulation success with D. silvestris males, which in turn was primarily due to a reduced success of reaching later stages of courtship. The time spent in copulation by hybrid males was intermediate between the two parental males. We studied aggression by observing the interactions of males of heterotypic pairs, both between the parental species and between the hybrids and parental males. A lack of aggressive interactions between D, silvestris males and D. heteroneura males in addition to the lack of courtship suggests that D. silvestris males do not respond to D. heteroneura individuals of either sex. Hybrid males were equally successful in winning fights with both D. silvestris and D. heteroneura males. These results indicate that the behavioral isolation between D. silvestris and D. heteroneura may be largely a consequence of the earliest stages of interactions. The two species may differ either in activity levels or in morphological or chemical traits that are important for species and mate recognition. The relatively high copulation and aggressive success of hybrids indicates that sexual selection against hybrids alone is unlikely to be a sufficient force to reduce gene flow and maintain species distinctions.