Ecotypic and genetic differentiation among native range and invasive populations of Queensland fruit fly
The Queensland fruit fly (Bactrocera tryoni Froggatt; Qfly) is a highly polyphagous pest which has substantially expanded its range during the twentieth century in association with the spread of horticulture. Its range now covers much of the east coast of Australia, including tropical, subtropical and temperate regions. It has also expanded westward in northern Australia and now overlaps with the native range of Bactrocera aquilonis, another fruit fly pest to which some authors accord full species status. Additionally it has invaded some Melanesian islands and new incursions are now regularly reported in areas of southern Australia that are still thought to remain free of permanent Qfly populations. This thesis aims to develop a better understanding of the intrinsic and extrinsic factors that contribute to Qfly invasiveness.
In the first part of my project I screened for variation in key environmental fitness traits among twelve populations from across the geographic range of this species and monitored changes in those traits during the domestication of eight of those populations. Significant variation was detected among the populations for heat, desiccation and starvation tolerance and wing length (as a measure of body size). All three stress tolerance phenotypes and wing length also changed significantly in certain populations during domestication, with a general trend for tolerance to decline in just a few generations of laboratory culture.
I then applied reduced representation genome-wide resequencing to a total of 359 individuals from 35 populations from across the species’ range. This uncovered significant genetic differentiation along an east-west transect across northern Australia, which likely reflects limited but bidirectional gene flow between B. tryoni and B. aquilonis. It also revealed some genetic differentiation following the southward expansion of B. tryoni, most of it associated with a move into previously marginal inland habitats. Two disjunct populations elsewhere in Australia and three on Melanesian islands were also clearly differentiated from the others, with the data suggesting their establishment from relatively few founders and subsequent isolation from other populations.
I then carried out reduced representation sequencing of a further 111 individual Qflies from 11 collections from recent incursions in areas in New Zealand, Tasmania and South Australia previously lacking permanent Qfly populations. Comparison of the results to the database from the 359 individuals above enabled me to identify the broad areas from which the incursion samples originated. I found that most but not all the incursion samples analysed originated in east coast Australia, but the data also showed there had been several more incursions events than previously known. Refinement of this approach, particularly through the use of full genome resequencing, should allow more precise identification of incursion sources.
Overall, this thesis provides valuable insights into Qfly ecology and population genetic history. In particular, I have shown genetic differentiation across the species range, including heritable ecotypic variation in climate stress tolerance. I have also shown that some of that tolerance is rapidly lost during domestication, which suggests ways in which mass rearing protocols might be modified to better preserve field vigour of sterile males used in Sterile Insect Technique releases. I have also developed and applied a new method for the identification of incursion origins which could help target future quarantine protocols towards areas presenting the greatest biosecurity risks.