Queensland Fruit Fly (Bactrocera tryoni - TRY) and Lesser Queensland Fruit Fly (B. neohumeralis - NEO) overlap in distribution over large areas of Queensland.  The range of TRY also includes northern coastal NSW and has spread to inland NSW and to more Southern regions of Australia.  In the 1960s, RC Lewontin and LC Birch put forward the hypothesis, based on the lack of detectable inviability in hybrids, that the Southern form of TRY might involve introgression from some NEO genes.  Contemporary phenotypic intermediates in the wild have been shown to be genetically non-hybrid.  However, genome sequencing data are consistent with at least one historical hybridisation event, although indicating that only around 1-2% of the genome is involved.  This conclusion is complicated by the extreme similarity of the two genomes in virtually every genome region investigated.

Many questions are left unanswered.  The chief of these relates to the question of how much hybridisation occurs in the areas of overlap, and how the two species can maintain their identity in the face of any such hybridisation.  The species isolation mechanism (time of day for mating) is a strong barrier to hybridisation, although it can be breached in the laboratory, with TRY (dusk mating) essentially dominant to NEO (day mating).  This suggests that NEO genes can be incorporated into TRY rather than vice versa.  The observed direction of apparent introgression in Southern TRY is thus in agreement with the genetic observations on inheritance of mating-time.