Close genetic similarity between two sympatric species of tephritid fruit fly reproductively isolated by mating time

Two sibling species of tephritid fruit fly, Bactrocera tryoni and B. neohum eralis, occur sympatrically throughout the range of B. neohumeralis in Aust ralia. Isolation between the two species appears to be maintained by a diff erence in mating time: B. tryoni mates at dusk, whereas B. neohumeralis mat es during the middle of the day. A morphological difference in humeral call us color also distinguishes the two species. Despite clear phenotypic evide nce that B. tryoni and B. neohumeralis are distinct species, genetic differ entiation as measured by four markers-nuclear DNA sequences from the white gene and the ribosomal internal transcribed spacer (ITS2), and mitochondria l DNA sequences from the cytochrome b (cytb) and cytochrome oxidase subunit II (COII) genes-is very small. Minor fixed differences occur in the ITS2 s equence, however, in all other cases the two species exhibit a high level o f shared polymorphic variation. The close genetic similarity suggests eithe r that speciation has occurred very rapidly and recently in the absence of any mitochondrial DNA sorting or that the sharing of polymorphisms is due t o hybridization or introgression. A third species within the tryoni complex , B. aquilonis, is geographically isolated. Bactrocera aquilonis is also ge netically very similar, but in this case there is clear differentiation for the mitochondrial loci. The three species form a group of considerable int erest for investigation of speciation mechanisms.