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.
Keywords: Helicoverpa armigera; Helicoverpa assulta; antennal lobes; intraspecific hybridization; olfactory sensory neurons; sex pheromone.