The larva and adult of Helicoverpa armigera use differential gustatory receptors to sense sucrose.

Almost all herbivorous insects feed on plants and use sucrose as a feeding stimulant, but the molecular basis of their sucrose reception remains unclear. Helicoverpa armigera as a notorious crop pest worldwide mainly feeds on reproductive organs of many plant species in the larval stage, and its adult draws nectar. In this study, we determined that the sucrose sensory neurons located in the contact chemosensilla on larval maxillary galea were 100-1000 times more sensitive to sucrose than those on adult antennae, tarsi, and proboscis. Using the Xenopus expression system, we discovered that Gr10 highly expressed in the larval sensilla was specifically tuned to sucrose, while Gr6 highly expressed in the adult sensilla responded to fucose, sucrose and fructose. Moreover, using CRISPR/Cas9, we revealed that Gr10 was mainly used by larvae to detect lower sucrose, while Gr6 was primarily used by adults to detect higher sucrose and other saccharides, which results in differences in selectivity and sensitivity between larval and adult sugar sensory neurons. Our results demonstrate the sugar receptors in this moth are evolved to adapt toward the larval and adult foods with different types and amounts of sugar, and fill in a gap in sweet taste of animals.

Identification and functional characterization of chemosensory genes in olfactory and taste organs of Spodoptera litura (Lepidoptera: Noctuidae).

The tobacco cutworm Spodoptera litura is one of the most destructive polyphagous crop pests. Olfaction and taste play a crucial role in its host plant selection and sexual communication, but the expression profile of chemosensory genes remains unclear. In this study, we identified 185 chemosensory genes from 7 organs in S. litura by transcriptome sequencing, of which 72 genes were published for the first time, including 27 odorant receptors (ORs), 26 gustatory receptors (GRs), 1 ionotropic receptor (IR), 16 odorant-binding proteins (OBPs), and 2 chemosensory proteins (CSPs). Phylogenetic analyses revealed that ORs, IRs, OBPs, and sensory neuron membrane proteins (SNMPs) were mainly expressed in antennae and sequence-conserved among Noctuidae species. The most differentially expressed genes (DEGs) between sexes were ORs and OBPs, and no DEGs were found in GRs. GR transcripts were enriched in proboscis, and the expression of sugar receptors was the highest. Carbon dioxide receptors, sugar receptor-SliuGR6, and bitter GRs-SlituGR43 and SlituGR66 had higher sequence identities between Noctuidae species. CSPs were broadly expressed in various organs, and SlituCSP13 was a DEG in adult antennae. The functional analysis in the Drosophila OR67d expression system found that SlituOR50, a receptor highly expressed in female antennae, is selectively tuned to farnesyl acetate. The results provide a solid foundation for understanding the molecular mechanisms by which chemosensory genes operate to elicit behavioral responses in polyphagous insects.

Functional analysis of pheromone receptor repertoire in the fall armyworm, Spodoptera frugiperda.

BACKGROUND: The fall armyworm, Spodoptera frugiperda (J. E. Smith), is a polyphagous moth species that is spreading all around the globe. It uses (Z)-9-tetradecenyl acetate (Z9-14:Ac) and (Z)-7-dodecenyl acetate (Z7-12:Ac) (100:3.9) as essential sex pheromone components. However, our understanding of the molecular basis of pheromone detection of S. frugiperda is still incomplete. RESULTS: Herein, we identified six PRs, i.e. SfruOR6, 11, 13, 16, 56, and 62, by transcriptome sequencing. Subsequently, we heterologously expressed them in Drosophila OR67d neurons and determined their response spectra with a large panel of sex pheromones and analogs. Among them, SfruOR13-expressing neurons strongly respond to the major sex pheromone component Z9-14:Ac, but also comparably to (Z,E)-9,12-tetradecadienyl acetate (Z9,E12-14:Ac) and weakly to (Z)-9-dodecenyl acetate (Z9-12:Ac). Both SfruOR56 and SfruOR62 are specifically tuned to the minor sex pheromone component Z7-12:Ac with varying intensities and sensitivities. In addition, SfruOR6 is activated only by Z9,E12-14:Ac, and SfruOR16 by both (Z)-9-tetradecenol (Z9-14:OH) and (Z)-9-tetradecenal (Z9-14:Ald). However, the OR67d neurons expressing SfruOR11 remain silent to all compounds tested, a phenomenon commonly found in the OR11 clade of Noctuidae species. Next, using single sensillum recording, we characterized four sensilla types on the antennae of males, namely A, B, C and D types that are tuned to the ligands of PRs, thereby confirming that S. frugiperda uses both SfruOR56 and SfruOR62 to detect Z7-12:Ac. Finally, using wind tunnel assay, we demonstrate that both Z9,E12-14:Ac and Z9-14:OH act as antagonists to the sex pheromone. CONCLUSION: We have deorphanized five PRs and characterized four types of sensilla responsible for the detection of pheromone compounds, providing insights into the peripheral encoding of sex pheromones in S. frugiperda.

Design of larval chemical attractants based on odorant response spectra of odorant receptors in the cotton bollworm.

Lepidopteran caterpillars rely on olfaction and gustation to discriminate among food sources. Compared to the larval gustation, the larval olfaction has been poorly investigated. To uncover the molecular basis of olfaction in Helicoverpa armigera larvae, we identified 17 odorant receptor (Or) genes in larval antennae and maxillae using transcriptome sequencing, and functionally identified the response spectra of seven Ors to ecologically relevant odorants. Innate behavioural responses of larvae to active odorants were evaluated in chemotaxis assays. Several odorant blends were formulated based on the Ors tuning spectra and caterpillar chemotaxis. A four-component blend strongly attracted H. armigera larvae, and cis-jasmone and 1-pentanol were identified as essential components. Localization analyses showed that the two Ors detecting these components (Or41 and Or52) were expressed in the same sensory neurons. This is the first evidence that Ors in a polyphagous caterpillar respond to odorants in a combinatorial manner. The design of attractants to target specific olfactory pathways may promote the development of new baits for pest management.

Inheritance of electrophysiological responses to leaf saps of host- and nonhost plants in two Helicoverpa species and their hybrids.

The polyphagous cotton bollworm Helicoverpa armigera (Hübner) and the oligophagous oriental tobacco budworm Helicoverpa assulta (Guenée) (Lepidoptera: Noctuidae) display contrasting heritable feeding preferences for cotton and pepper leaves. In this study, electrophysiological response patterns to cotton and pepper leaf saps in gustatory sensilla styloconica on the maxillae of these two species, their reciprocal F1 hybrids, and backcrossed lines were investigated using the tip recording technique. The identity of the neurons responding to the two leaf saps has been established using action potential waveform analysis. The two plant leaf saps elicited neural activity in at least six of the eight taste neurons innervating the lateral and medial sensilla styloconica of the parental species and crosses. Discriminant analysis of this multineural input predicted that correct classification occurred in 87 - 92% of the cases. Differences in taste neuron responses between insect lines to the two plant saps were consistent with differences in feeding preference behaviors. Comparisons of taste neuron response patterns of parental species, F1 hybrids and backcrosses indicate that autosomal loci contributed to the difference in gustatory response patterns between the two Helicoverpa species with the H. armigera derived alleles being partly dominant to those carried by H. assulta. These findings contribute to the understanding of gustatory codes for preference and provide insight into taste evolution of lepidopteran insects.