Polyandry and paternity in the Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae)
The Queensland fruit fly, Bactrocera tryoni (Froggatt) (Qfly) (Tephritidae), is one of Australia’s most economically important pests. Over the past 40 years pre- and postharvest management of Qfly has relied heavily on organophosphate insecticides, but recent restrictions in the use of key organophosphates has resulted in a search for alternative management options. One of the most effective and sustainable options to manage Qfly is sterile insect technique (SIT). SIT relies on mass release of sterile males to mate with wild females to reduce production of viable offspring and thereby reduce the pest population. SIT can be improved through a deep knowledge of preand post-copulatory reproductive biology. This thesis focuses on aspects of polyandry and post-copulatory sexual selection that have implications for Qfly SIT.
Studies of post-copulatory sexual selection in Qfly often require sperm quantification. I developed a PCR-based Y-specific sperm quantification method that enabled increased accuracy, throughput, and sample processing flexibility compared to previous methods. Due mainly to enhanced accuracy the new method has enabled revised estimates of key sperm storage parameters, including lower levels of sperm storage asymmetry, positive correlation between sperm transfer and copula duration and a much higher number of sperm stored by females in single mating trials.
An attempt was made to further enhance the sensitivity of sperm quantification. The developed PCR-based method cannot reliably detect sperm numbers below 50 and a Droplet Digital PCR (ddPCR) technology was used with the same Y-specific marker and sperm DNA extraction protocol to increase detection sensitivity. Hurdles such as rain formation, high standard deviation among replicates, and low droplet counts prevented finalization of this work; however, this work serves as a starting point for future studies aimed at increasing sperm quantification sensitivity.
Female remating sets the stage for sperm competition and cryptic female choice. Patterns of sperm storage were studied in twice-mated female Qfly using PCR and capillary electrophoresis-based technologies. Significantly fewer sperm were transferred by the second male and the copula duration was also shorter. Asymmetry between the spermathecae in storage of sperm from the first mate affects distribution of sperm stored from second mates.
Patterns of sperm use by twice-mated female Qfly was assessed using two phenotypic lines including a normal and a CRISPR-Cas9-based yellow line. Mating competitive-related parameters were shown to be comparable between yellow and normal lines. P2, proportion of progeny sired by the second male, was close to 0.5 immediately following the second mating and decreased as females aged. Overall first male sperm precedence was recorded for Qfly and a mechanism of initial sperm stratification followed by later mixing is suggested.
Rates of polyandry in two wild populations of Qfly from QLD and NSW were assessed via genotyping of stored sperm for ten microsatellite loci. Results indicated on average, conservative estimates of 1.27 and 1.8 and probabilistic estimates of 1.54 and 2.64 mates per female for QLD and NSW populations respectively. Implications of these results in the context of SIT are discussed.
