Abstract
Hybridization of sympatric closely related species may sometimes lead to introgression and speciation. The sister species Helicoverpa armigera and Helicoverpa assulta both use (Z)-11-hexadecenal and (Z)-9-hexadecenal as sex pheromone components but in reversed ratios. Female H. armigera and male H. assulta could hybridize and produce fertile male hybrids, which can then backcross with females of the two parent species to get backcross lines in the laboratory. In this study, we compared the olfactory responses to pheromone compounds in the periphery and in the antennal lobes (ALs) of males of the two species, as well as of their hybrids and backcrosses. Single-sensillum recordings were carried out to explore characteristics of male-specific sensilla on the antennae, and in vivo calcium imaging combined with digital 3D-reconstruction was used to describe what happens in the macroglomerular complex (MGC) of the AL. The results show that the population ratio of the two male-specific types of olfactory sensory neurons responding to two sex pheromone components are controlled by a major gene, and that the allele of H. armigera is dominant. Consistently, the study of the representative areas activated by sex pheromone components in the ALs further support the dominance of H. armigera. However, the topological structure of the MGC in the hybrid was similar but not identical to that in H. armigera. All subtypes of male-specific sensilla identified in the two species were found in the male hybrids and backcrosses. Moreover, two new subtypes with broader response spectra (the expanded A subtype and the expanded C subtype) emerged in the hybrids. Based on the inheritance pattern of the pheromone sensory system, we predict that when hybridization of female H. armigera and male H. assulta occurs in the field, male hybrids would readily backcross with female H. armigera, and introgression might occur from H. assulta into H. armigera through repeated backcrossing.
Highlights
Hybridization is defined as interspecific mating and produces viable offspring
The two species could be successfully hybridized in the laboratory (Wang and Dong, 2001; Wang et al, 2004; Zhao et al, 2006) and the fertile hybrids provide us with an unique opportunity to study the genetic basis of sex pheromone detection
We found that the inheritance of the population ratio between the two types of sensilla tuned to two pheromone components is controlled by a major gene, and the allele of H. armigera is dominant
Summary
Hybridization is defined as interspecific mating and produces viable offspring. It has been reported to occur in many species of both plants and animals (Mallet, 2005; Baack and Rieseberg, 2007). On the other hand, when reproductive isolation does not take place because of lack of ecological and spatial isolation, infertile hybrids, or other reasons (McCarthy et al, 1995; Buerkle et al, 2000), hybrids might backcross with their parents. Such a gene flow (introgression) through repeated backcrossing could be an important evolutionary drive (Baack and Rieseberg, 2007; Mallet, 2007). Interspecies mating and generation of hybrids often occur in related insect species, but the fate of hybrids has been largely ignored (Hansson et al, 1987; Roelofs et al, 1987; Baker et al, 2006; Vickers, 2006a,b; Domingue et al, 2008)
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