Abstract
BackgroundBactrocera 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.ResultsOur 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.ConclusionWe 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.
Highlights
Bactrocera tryoni and Bactrocera neohumeralis mate asynchronously; the former mates exclusively around dusk while the latter mates during the day
Genetic architecture of mating time variation and callus colour based on backcross analysis Analysis of a backcross population confirmed the phenotypic divergence in the two parental species and dominance relationship of the phenotypes, and revealed interspecific differences in females, linkage between traits and the relative genetic complexity of each trait
Amongst the mates of B. tryoni males at dusk, approximately half (18/39 or 46.2%) were B. neohumeralis, suggesting that B. tryoni males did not distinguish between the two types of females, and that females of both species could mate at dusk
Summary
Bactrocera tryoni and Bactrocera neohumeralis mate asynchronously; the former mates exclusively around dusk while the latter mates during the day. Maintenance of species boundaries between hybridisable species in sympatry requires ongoing reproductive isolating mechanisms to impede interspecific gene flow. In addition to host shifts and pheromonal variation, temporal separation of potentially inter-fertile populations is another wellrecognised premating barrier to reduce gene flow [2]. The timescale of such allochronic delimitation of life cycle events (e.g. reproductive activities) can range from different times of a day, to between seasons, or even between years [2]. Intraspecific variation in reproductive timing has evolved in Anastrepha fraterculus where geographically separated ecotypes have mating receptivity times a few hours apart [6]. Variation in mating time within tephritid species can be artificially induced in the laboratory over time, through selection for different developmental time, e.g. in Zeugodacus cucurbitae [7] or via long term laboratory domestication, e.g. in Bactrocera oleae [8]
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