In recent years, considerable progress has been made in understanding the molecular mechanisms underlying olfaction in insects. Because of the diverse nature of the gene families involved, this process has largely relied on genomic data. As a consequence, studies have focused on a small subset of species with extensive genomic information. For Lepidoptera, a large order historically crucial to olfactory research, this circumstance has mostly limited advances to the domesticated species Bombyx mori , with some progress in the noctuid Heliothis virescens based on a nonpublic partial genome database. Because of the limited behavioral repertoire and nonexistent ecological importance of Bombyx , molecular data on the tobacco hornworm Manduca sexta are of utmost importance, especially with regards to its position as a classical olfactory model and its complex natural behavior. Here we present the use of transcriptomic and microarray data to identify members of the main olfactory gene families of Manduca. To assess the quality of our data, we correlate information on expressed receptor genes with detailed morphological data on the antennal lobe. Finally, we compare the expression of the near-complete transcript sets in male and female antennae.

The olfactory sense detects a plethora of behaviorally relevant odor molecules; gene families involved in olfaction exhibit high diversity in different animal phyla. Insects detect volatile molecules using olfactory (OR) or ionotropic receptors (IR) and in some cases gustatory receptors (GRs). While IRs are expressed in olfactory organs across Protostomia, ORs have been hypothesized to be an adaptation to a terrestrial insect lifestyle. We investigated the olfactory system of the primary wingless bristletail Lepismachilis y-signata (Archaeognatha), the firebrat Thermobia domestica (Zygentoma) and the neopteran leaf insect Phyllium siccifolium (Phasmatodea). ORs and the olfactory coreceptor (Orco) are with very high probability lacking in Lepismachilis; in Thermobia we have identified three Orco candidates, and in Phyllium a fully developed OR/Orco-based system. We suggest that ORs did not arise as an adaptation to a terrestrial lifestyle, but evolved later in insect evolution, with Orco being present before the appearance of ORs.

The enormous capacity of the male silkmoth Bombyx mori in recognizing and discriminating bombykol and bombykal is based on distinct sensory neurons in the antennal sensilla hairs. The hydrophobic pheromonal compounds are supposed to be ferried by soluble pheromone-binding proteins (PBPs) through the sensillum lymph toward the receptors in the dendritic membrane. We have generated stable cell lines expressing the candidate pheromone receptors of B. mori, BmOR-1 or BmOR-3, and assessed their responses to hydrophobic pheromone compounds dissolved by means of dimethyl sulfoxide. BmOR-1–expressing cells were activated by bombykol but also responded to bombykal, whereas cells expressing BmOR-3 responded to bombykal only. In experiments employing the B. mori PBP, no organic solvent was necessary to mediate an activation of BmOR-1 by bombykol, indicating that the PBP solubilizes the hydrophobic compound. Furthermore, the employed PBP selectively mediated a response to bombykol but not to bombykal, supporting a ligand specificity of PBPs. This study provides evidence that both distinct pheromone receptors and PBPs play an important role in insect pheromone recognition.

The remarkable responsiveness of male moths to female released pheromones is based on the extremely sensitive and selective reaction of highly specialized sensory cells in the male antennae. These cells are supposed to be equipped with male-specific receptors for pheromonal compounds, however, the nature of these receptors is still elusive. By using a combination of genomic sequence analysis and cDNA-library screening, we have cloned various cDNAs of the tobacco budworm Heliothis virescens encoding candidate olfactory receptors. A comparison of all identified receptor types not only highlighted their overall high degree of sequence diversity but also led to the identification of a small group of receptors sharing >40% identity. In RT-PCR analysis it was found that distinct members of this group were expressed exclusively in the antennae of male moths. In situ hybridization experiments revealed that the male-specific expression of these receptor types was confined to antennal cells located beneath sensillar hair structures (sensilla triochoidea), which have been shown to contain pheromone-sensitive neurons. Moreover, two-color double in situ -hybridization approaches uncovered that cells expressing one of these receptor types were surrounded by cells expressing pheromone-binding proteins, as expected for a pheromone-sensitive sensillum. These findings suggest that receptors like Heliothis receptor 14–16 (HR14–HR16) may render antennal cells responsive to pheromones.

The European spruce bark beetle, Ips typographus, and the North American mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae: Scolytinae), are severe pests of coniferous forests. Both bark beetle species utilize aggregation pheromones to coordinate mass-attacks on host trees, while odorants from host and non-host trees modulate the pheromone response. Thus, the bark beetle olfactory sense is of utmost importance for fitness. However, information on the genes underlying olfactory detection has been lacking in bark beetles and is limited in Coleoptera. We assembled antennal transcriptomes from next-generation sequencing of I. typographus and D. ponderosae to identify members of the major chemosensory multi-gene families.