Differential pheromone profile as a contributor to premating isolation between two sympatric sibling fruit fly species.

Bactrocera tryoni (Froggatt) and Bactrocera neohumeralis (Hardy) are sibling fruit fly species that are sympatric over much of their ranges. Premating isolation of these close relatives is thought to be maintained in part by allochrony-mating activity in B. tryoni peaks at dusk, whereas in B. neohumeralis, it peaks earlier in the day. To ascertain whether differences in pheromone composition may also contribute to premating isolation between them, this study used solid-phase microextraction and gas chromatography-mass spectrometry to characterize the rectal gland volatiles of a recently collected and a more domesticated strain of each species. These glands are typical production sites and reservoirs of pheromones in bactrocerans. A total of 120 peaks were detected and 50 were identified. Differences were found in the composition of the rectal gland emissions between the sexes, species, and recently collected versus domesticated strains of each species. The compositional variation included several presence/absence and many quantitative differences. Species and strain differences in males included several relatively small alcohols, esters, and aliphatic amides. Species and strain differences in females also included some of the amides but additionally involved many fatty acid esters and 3 spiroacetals. While the strain differences indicate there is also heritable variation in rectal gland emissions within each species, the species differences imply that compositional differences in pheromones emitted from rectal glands could contribute to the premating isolation between B. tryoni and B. neohumeralis. The changes during domestication could also have significant implications for the efficacy of Sterile Insect Technique control programs.

Separating two tightly linked species-defining phenotypes in Bactrocera with hybrid recombinant analysis.

BACKGROUND: Bactrocera tryoni and Bactrocera neohumeralis mate asynchronously; the former mates exclusively around dusk while the latter mates during the day. The two species also differ in the colour of the post-pronotal lobe (callus), which is predominantly yellow in B. tryoni and brown in B. neohumeralis. We have examined the genetic relationship between the two characters in hybrids, backcrosses and multigeneration hybrid progeny. RESULTS: Our analysis of the mating time of the parental species revealed that while B. tryoni mate exclusively at dusk, B. neohumeralis females pair with B. neohumeralis males during the day and with B. tryoni males at dusk. We found considerable variance in mating time and callus colour among hybrid backcross individuals of both sexes but there was a strong although not invariant trend for callus colour to co-segregate with mating time in both sexes. To genetically separate these two phenotypes we allowed the interspecific F1 hybrids to propagate for 25 generations (F25) without selection for mating time or callus colour, finding that the advanced hybrid population had moved towards B. tryoni phenotypes for both traits. Selection for day mating in replicate lines at F25 resulted in significant phenotypic shifts in both traits towards B. neohumeralis phenotypes in F26. However, we were unable to completely recover the mating time profile of B. neohumeralis and relaxation of selection for day mating led to a shift back towards dusk mating, but not yellow callus colour, by F35. CONCLUSION: We conclude that the inheritance of the two major species-defining traits is separable but tightly linked and involves more than one gene in each case. It also appears that laboratory conditions select for the B. tryoni phenotypes for mating time. We discuss our findings in relation to speciation theory and the likely effects of domestication during the generation of mass release strains for sterile insect control programmes.