The Bactrocera tryoni complex of tephritid flies in Australia

Full text: The tryoni complex of tephritid fruit flies consists of three named species, all endemic to Australia. The sibling species Bactrocera tryoni and B. neohumeralis are serious pests of horticulture in eastern Australia. The third named species of the tryoni complex, B. aquilonis, occurs in northern Australia, geographically isolated from B. tryoni and B. neohumeralis, and was not considered a pest. B. tryoni and B. neohumeralis have an overlapping geographic range and the same or extremely similar host range, but differ in bioclimatic potential and hence pest status. Since the introduction of horticulture to Australia in the mid 19th century, B. tryoni, but not B. neohumeralis, has spread south from an original endemic habitat - the rainforests of eastern Australia - into temperate fruit production areas in the south of the country. In northern Australia, considerable numbers of flies have been trapped in commercial fruit crops in recent years. These flies are indistinguishable from B. aquilonus, calling into question the pest status of the endemic B. aquilonus. We are undertaking molecular studies with the aim of identifying the genetics of species differences and how they relate to pest status. The three species are all extremely closely related both morphologically and genetically. B. tryoni and B. neohumeralis are distinguished by a colour difference of the humeral calli, and a difference in time of mating acts as a mating isolation mechanism. A set of 30 microsatellites reveals differentiation between B. tryoni and B. neohumeralis at the level of allele frequency, but no fixed microsatellite allele differences have been identified. Sharing of polymorphisms in coding and non-coding regions is common indicating continued genetic exchange between the two species. Wild flies with intermediate callus colour were long thought to represent hybrids, however microsatellite analysis indicates that these 'hybrids' are intraspecies variants. Population analysis of B. tryoni shows stable subdivision, over many years, within the east coast distribution, indicating the feasibility of area-wide eradication of outlying populations and outbreaks. Bait sprays, cover sprays and SIT are all used to control infestations, with SIT the method of choice as an efficient and environment-benign protocol. B. aquilonus is morphologically identical to B. tryoni, and like B. tryoni mates at dusk. Hybrid flies are viable and fertile. It is unknown whether infestations in northern Australia are B. tryoni, hybrids between B. tryoni and B. aquilonus or a 'noxious' strain of B. aquilonus. Population analysis using microsatellite and mitochondrial DNA markers failed to detect any evidence of spatial structuring in northern, wild-caught flies, suggesting the presence of only one species in the region. An understanding of how pest status is determined and maintained is being addressed through the characterisation of genes of the circadian clock that enable the fly to adapt to environmental cues. Using a candidate gene approach and differential screening we have isolated two transcripts that show quantitative differences that correlate with mating time. The results support the hypothesis that the circadian system in the flies has a role in the mating isolation mechanism. The B. tryoni complex of tephritids is an ideal model system for identifying the genetics of species differences. We believe that an understanding of species differences and how they relate to pest status will be useful in the future to the effective delivery of sophisticated pest control measures. (author)