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

We investigated the role that courtship and aggressive interactions ma y have for the maintenance of reproductive isolation between Drosophil a silvestris and D. heteroneura. We examined the behavioral bases of r eproductive isolation between the parental species and we examined the courtship success of each sex of both reciprocal F-1 hybrids when pai red with the parental species. We found reduced copulation success amo ng heterotypic parental pairs compared to homotypic pairs, which was p rimarily due to the lack of courtship initiation between D. silvestris males and D. heteroneura females. When hybrid males from both recipro cal crosses were paired with parental females their copulation success es were not significantly different from that of parental males. In co ntrast, hybrid females from both crosses had reduced copulation succes s with D. silvestris males, which in turn was primarily due to a reduc ed success of reaching later stages of courtship. The time spent in co pulation by hybrid males was intermediate between the two parental mal es. We studied aggression by observing the interactions of males of he terotypic pairs, both between the parental species and between the hyb rids and parental males. A lack of aggressive interactions between D, silvestris males and D. heteroneura males in addition to the lack of c ourtship suggests that D. silvestris males do not respond to D. hetero neura individuals of either sex. Hybrid males were equally successful in winning fights with both D. silvestris and D. heteroneura males. Th ese results indicate that the behavioral isolation between D. silvestr is and D. heteroneura may be largely a consequence of the earliest sta ges of interactions. The two species may differ either in activity lev els or in morphological or chemical traits that are important for spec ies and mate recognition. The relatively high copulation and aggressiv e success of hybrids indicates that sexual selection against hybrids a lone is unlikely to be a sufficient force to reduce gene flow and main tain species distinctions.