Abstract

Two sympatric, distantly related Drosophila species, D. melanogaster and D. affinis have been shown to interact
sexually. Mature D. melanogaster males perform vigorous courtship in response to both virgin and mated D.
affinis females, but perform no courtship in response to virgin females of another sympatric species, D.
immigrans. To determine whether inter-specific courtship might interfere with mating success and fitness, D.
affinis females were used as ‘interference sex objects’ in tests where D. melanogaster males had access to
con-specific females. In these tests the presence of D. affinis females was shown to significantly reduce the
mating success of D. melanogaster males. In comparison, the presence of mated D. melanogaster females or
virgin D. immigrans females had no impact on the mating success of D. melanogaster males in similar tests.

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