Abstract
Two sympatric species Helicoverpa armigera and Helicoverpa assulta use (Z)-11-hexadecenal and (Z)-9-hexadecenal as sex pheromone components in reverse ratio. They also share several other pheromone gland components (PGCs). We present a comparative study on the olfactory coding mechanism and behavioral effects of these additional PGCs in pheromone communication of the two species using single sensillum recording, in situ hybridization, calcium imaging, and wind tunnel. We classify antennal sensilla types A, B and C into A, B1, B2, C1, C2 and C3 based on the response profiles, and identify the glomeruli responsible for antagonist detection in both species. The abundance of these sensilla types when compared with the number of OSNs expressing each of six pheromone receptors suggests that HarmOR13 and HassOR13 are expressed in OSNs housed within A type sensilla, HarmOR14b within B and C type sensilla, while HassOR6 and HassOR16 within some of C type sensilla. We find that for H. armigera, (Z)-11-hexadecenol and (Z)-11-hexadecenyl acetate act as behavioral antagonists. For H. assulta, instead, (Z)-11-hexadecenyl acetate acts as an agonist, while (Z)-9-hexadecenol, (Z)-11-hexadecenol and (Z)-9-hexadecenyl acetate are antagonists. The results provide an overall picture of intra- and interspecific olfactory and behavioral responses to all PGCs in two sister species.
Highlights
The response profiles of the OSNs are mainly determined by which odorant receptors (ORs) they express
Based on the response profiles of their respective OSNs, the sensilla activated by PGCs in the 30-60 annuli of antennal flagella were first classified into A, B, C types consistently with previous reports[10,11]
The subtype B1 responded to Z9-14:Ald only, while subtype B2 responded to Z9-14:Ald as well as to an alcohol (Z11-16:OH in H. armigera and Z9-16:OH in H. assulta) (Figs 1C–F and 2C–F)
Summary
The response profiles of the OSNs are mainly determined by which odorant receptors (ORs) they express. The AL structure of Heliothine species is sexually dimorphic, embodied by a macroglomerular complex (MGC) This structure is located at the input area of ALs of males and receives information from OSNs tuned to pheromone components and related compounds[25,26,27,28]. In order to elucidate the role of the other PGCs in pheromone communication and interspecific behavioral isolation between H. armigera and H. assulta, and assign different PRs to OSNs and types of sensilla tuned to PGCs, we firstly performed a detailed classification of the types of antennal sensilla based on the response profiles of their housed OSNs to PGCs, and evaluated the population size of each type of sensilla in the two species; secondly we used in situ hybridization to determine the expression levels of six PRs and compared the values with the numbers of the associated OSNs in each species along the male antennae; thirdly we used in vivo optical imaging to study the transmission of peripheral input signals to antennal lobes in the brains of both species; we clarified the behavioral significance of PGCs in H. armigera and H. assulta with respect to pheromone communication and species isolation
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