Ionotropic receptors in the turnip moth Agrotis segetum respond to repellent medium-chain fatty acids

Abstract

Background In insects, airborne chemical signals are mainly detected by two receptor families, odorant receptors (ORs) and ionotropic receptors (IRs). Functions of ORs have been intensively investigated in Diptera and Lepidoptera, while the functions and evolution of the more ancient IR family remain largely unexplored beyond Diptera. Results Here, we identified a repertoire of 26 IRs from transcriptomes of female and male antennae, and ovipositors in the moth Agrotis segetum. We observed that a large clade formed by IR75p and IR75q expansions is closely related to the acid-sensing IRs identified in Diptera. We functionally assayed each of the five AsegIRs from this clade using Xenopus oocytes and found that two receptors responded to the tested ligands. AsegIR75p.1 responded to several compounds but hexanoic acid was revealed to be the primary ligand, and AsegIR75q.1 responded primarily to octanoic acid, and less so to nonanoic acid. It has been reported that the C6-C10 medium-chain fatty acids repel various insects including many drosophilids and mosquitos. We show that the C6-C10 medium-chain fatty acids elicited antennal responses of both sexes of A. segetum, while only octanoic acid had repellent effect to the moths in a behavioral assay. In addition, using fluorescence in situ hybridization, we demonstrated that the five IRs and their co-receptor AsegIR8a are not located in coeloconic sensilla as found in Drosophila, but in basiconic or trichoid sensilla. Conclusions Our results significantly expand the current knowledge of the insect IR family. Based on the functional data in combination with phylogenetic analysis, we propose that subfunctionalization after gene duplication plays an important role in the evolution of ligand specificities of the acid-sensing IRs in Lepidoptera.

Ionotropic receptor 8a is involved in the attraction of Helicoverpa armigera to acetic acid

“…Interestingly, in Microplitis mediator , a ternary expression pattern was found, whereby IR8a co‐expressed with two IRs (IR64a1 and IR64a2) was necessary for sensing acids, indicating the co‐expression pattern might be more complicated than that of ORs, and thus may require further study (Shan et al ., 2019). In addition, IR8a co‐expressed with IR75p.1 responds to acids and aldehydes with exclusively C5–C7 straight chains, and with IR75q.2 responds to octanoic acid in Agrotis segetum (Hou et al ., 2021). Taken together, we can then use the Xenopus oocyte expression system to identify the specific IRs of H. armigera that respond to acetic acid.…”

Sex- and tissue-specific expression of chemosensory receptor genes in a hawkmoth

“…Seven of these, MsexIR1.1, MsexIR1.2, MsexIR4, MsexIR75p.1, MsexIR75p.2, MsexIR75q.1, and MsexIR87a putatively encode IRs nested within the antennal IRs and are considered to have an olfactory function (Yin et al, 2021). All except MsexIR87a belong to a monophyletic group of putative acid-detectors (Hou et al, 2022). This group includes two acid-sensing IRs of Agrotis segetum, AsegIR75q.1, which is tuned specifically to octanoic acid and AsegIR75p.1, which has a broader tuning and detects acids, aldehydes and alcohols.…”

“…This group includes two acid-sensing IRs of Agrotis segetum, AsegIR75q.1, which is tuned specifically to octanoic acid and AsegIR75p.1, which has a broader tuning and detects acids, aldehydes and alcohols. Two Lepidopteraspecific IRs, MsexIR7d.3 and MsexIR100d, lie among the divergent IRs (Yin et al, 2021;Hou et al, 2022), which are defined as IRs that are not expressed in the antennae but in other tissues (Croset et al, 2010;Yin et al, 2021). However, the expression pattern of MsexIR7d.3 resembles that of its A. segetum ortholog AsegIR7d.3, which is highly expressed in the antennae (Hou et al, 2022).…”

“…Two Lepidopteraspecific IRs, MsexIR7d.3 and MsexIR100d, lie among the divergent IRs (Yin et al, 2021;Hou et al, 2022), which are defined as IRs that are not expressed in the antennae but in other tissues (Croset et al, 2010;Yin et al, 2021). However, the expression pattern of MsexIR7d.3 resembles that of its A. segetum ortholog AsegIR7d.3, which is highly expressed in the antennae (Hou et al, 2022). The function of receptors in the IR7d subfamily is unknown; however, the phylogenetically related DmelIR7a is expressed in neurons of the gustatory organs of larval and adult D. melanogaster (Croset et al, 2010), suggesting a gustatory function for these receptors.…”

“…We detected MsexIR8a transcripts in only one sample of a virgin female, indicating that MsexIR8a is expressed in very low levels in the proboscis, and PCR amplification would be a better technique for detecting this expression. The proboscis expression of genes of three IRs from the "expanded acid-sensing cluster" (MsexIR1.2, MsexIR4 and MsexIR75q.1) suggests that the proboscis could detect volatile carboxylic acids (Hou et al, 2022); however, based on the functional characterization of IRs from this cluster in the turnip moth A. segetum, such acid-sensing would require expression of the co-receptor IR8a.…”