The rectal gland chemistries of two species of bactroceran fruit flies

Bactrocera tryoni and Bactrocera neohumeralis are sympatric sibling species of fruit fly. Hybrids have never been confirmed between them in the field, even though they mate and produce fertile progeny in the laboratory. This is likely at least partly due to their allochrony; B. tryoni mates at dusk and B. neohumeralis around midday. However, B. tryoni mating is also influenced by volatiles secreted from males’ rectal glands. This thesis investigates the potential for such volatiles to contribute to assortative mating within and reproductive isolation between the species. Characterization of the molecular basis of mating behaviour could also have applied benefits, given the pest status of the two species, and the priority of the Australian horticultural industry to find non-pesticidal control methods. 

My first results chapter first uses solid phase microextraction coupled with gas chromatography-mass spectrometry (SPME GC-MS) to expand the inventory of known B. tryoni rectal gland volatiles. Many peaks show presence/absence differences between the two sexes. I then use gas chromatography coupled with flame ionization detection (GC-FID) to determine the relative abundances of over a hundred of the peaks. In addition to those showing presence/absence differences I find many other peaks show quantitative differences between the sexes and some also show differences due to mating history. Thus, many peaks could contribute to sex pheromone function in this species. 

My second results chapter screens for genetic variation affecting the GC-FID peaks in B. tryoni. Several isofemale lines recently established from diverse localities across its range are compared, along with a long established laboratory line. Differences are found for 26 of the peaks, many of them between the isofemale lines as a group and the domesticated line but several also just among the isofemale lines. The isofemale line variation includes differences due to source localities. This supports other evidence for ecotypic variation between B. tryoni populations and could form the basis for assortative mating and eventually further speciation. Many of the differences among lines are not correlated with one another, suggesting multiple genes are involved. 

My third results chapter compares the rectal gland emissions of male B. tryoni and B. neohumeralis by SPME GC-MS. Nearly 50 of over 100 peaks screened are identified. Many show presence/absence differences and many more show quantitative differences between the species, with many differences also between recently collected and domesticated strains of each species. In some cases, the same peaks are involved in the two species but in other cases they are not. I also find many of the species differences are carried through to whole male emissions. None of the peaks in either species individually show diurnal patterns of abundance but the total areas of all the peaks do so, and in each species, they correspond to their respective cycles in mating activity. 

I conclude that there is abundant divergence in rectal gland profiles between the species which could contribute to their pre-zygotic isolation, and also abundant polymorphism within the species that could provide the basis for the evolution of assortative mating within each species. The challenge now is to develop bioassay workflows to determine which of the many candidate peaks are directly involved in shared and species-specific pheromone functions.