Ionotropic receptors in the turnip mothAgrotis segetumrespond to repellent medium-chain fatty acids

Abstract

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. 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.2 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. Our GC-EAD recordings showed that 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 behavioural assay. In addition, using fluorescence in situ hybridization, we demonstrated that AsegIR75q.2 and its co-receptor AsegIR8a are not located in coeloconic sensilla as found in Drosophila, but in basiconic or trichoid sensilla. These functional data in combination with our phylogenetic analysis suggest that subfunctionalization of the acid-sensing IRs 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.…”

Transcriptome Analysis and Characterization of Chemosensory Genes in the Forest Pest, Dioryctria abietella (Lepidoptera: Pyralidae)

“…In insects, ORs are expressed in sensilla basiconica and sensilla trichoidea separately responsible for general odorants (i.e., general ORs) and sex pheromones (i.e., pheromone receptors, PRs) (Krieger et al, 2009;de Fouchier et al, 2017;Guo et al, 2020), while olfactory sensory neurons expressing IRs are distributed in sensilla coeloconica sensitive to amines, acids, alcohols and aldehydes (Yao et al, 2005;Silbering et al, 2011;Rytz et al, 2013). Beyond olfaction, IRs are dedicated to the detection of humidity, temperature or tastants (Stewart et al, 2015;Enjin et al, 2016;Knecht et al, 2016;Hou et al, 2021). Compared with ORs and IRs, this GR gene repertoire possesses more variable gene numbers, ranging from 45 in Manduca sexta to 237 in Spodoptera litura (Koenig et al, 2015;Cheng T. et al, 2017).…”