Bactrocera tryoni (Froggatt) and B. neohumeralis (Hardy) (Diptera: Tephritidae) are sympatric species which hybridise readily in the laboratory yet remain distinct in the field. B. tryoni mates only at dusk and B. neohumeralis mates only during the day, but hybrids can mate at both times. We investigated the inheritance of mating time in successively backcrossed hybrid stocks to establish whether mating with either species is more likely. The progeny of all backcrosses to B. tryoni mated only at dusk. The majority of the progeny of the first and a minority of the progeny of the second backcross to B. neohumeralis also mated at dusk, but the third successive B. neohumeralis backcross produced flies that mated only during the day. This trend towards dominance of the B. tryoni trait was also reflected in a diagnostic morphological character. We discuss the possible genetic background for these phenomena and propose that unidirectional gene flow might explain how the two species remain distinct in the face of natural hybridisation.
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