1 The tephritid fruit fly Zeugodacus tau is a serious agriculture pest, especially of cucurbits in Thailand, where it is classified as a complex species of nine siblings (Z. tau A, B, C, D, E, F, G, I and J). 2 Based on single-strand conformation polymorphism (SSCP) analysis, 33 distinct mitochondrial cytochrome c oxidase subunit I (mtCOI) haplotypes were found in three sibling species, Z. tau A, C and J, in Thailand. Genetic structure analysis of the population revealed relatively large genetic differences among these sibling species, reflecting a tendency toward speciation. 3 The maximum likelihood tree and median-joining network reconstructed from the mtCOI sequences of 33 haplotypes supported the separation of these three sibling species within the Z. tau complex. 4 The three-level hierarchical analysis of molecular variance based on geographical regions and the Mantel test for isolation-by-distance showed significant genetic structuring of the Z. tau complex in Thailand. 5 However, the genetic differentiation among geographical groups was rather low, which may be reasonably explained by natural and human mediated dispersal, as well as host associated differentiation. 6 The SSCP analysis on the population genetic structure of Z. tau species complex in the present study should benefit pest management.

Diachasmimorpha longicaudata is an Opiinae parasitoid used to control tephritid fruit flies, which cause tremendous economic losses of fruits worldwide. In Thailand, D. longicaudata is classified as three sibling species, DLA, DLB and DLBB, based on the morphological and biological species concepts but their genetic variation has not been studied. Therefore, we investigated the genetic differentiation of the mitochondrial COI gene to clarify the ambiguous taxonomy of this species complex. The 603-bp COI region was sequenced from laboratory-bred colonies and field-collected specimens from seven locations representing five geographical regions in Thailand. DLA was associated with the host Bactrocera correcta while DLB and DLBB were associated with Bactrocera dorsalis. The interspecific nucleotide differences of COI sequences among the three groups ranged from 6.70% to 7.62% (Kimura 2-parameter distance), which adequately separates species complexes within the order Hymenoptera and supports the current sibling species classification. The neighbor joining, maximum likelihood and consensus Bayesian phylogenetic trees constructed from COI sequences revealed that the three sibling species of laboratory and field-collected D. longicaudata are monophyletic with 100% support. The high genetic variation and molecular phylogeny of the COI sequences were shown to discriminate between the D. longicaudata species examined in this study.

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The population structure of a fruit fly, Zeugodacus tau, was studied by a Single-Strand Conformation Polymorphism (SSCP) technique. Twenty haplotypes of Cytochrome Oxidase unit I (COI) sequence were found in flies collected from Southern Thailand. The phylogenetic tree and haplotype network revealed gene flow across a large geographic range. With the aid of winds, their gene flow diminished population structure. The population size of Z. tau in Southern Thailand seemed to be large and stable, but the other populations in some locations had experienced a bottleneck effect, leading to local genetic differentiation. Fruit flies from the eastern areas had large effective population sizes, whereas the populations from other areas were smaller. This pattern matched the ecological niche centroid model, in which fruit flies disperse from high population areas to lower ones.