Functional conservation of IR75q.2 in the recognition of volatile acids and aldehydes in two Spodoptera species.

Ionotropic receptors (IRs) serve as important chemosensory receptors for the perception of diverse ecological volatiles in insects, but their specific functions in Lepidoptera remain largely unexplored. This study systematically compared the ligand recognition profiles and key binding residues of the highly expressed IR75q.2 receptor in two Spodoptera species. Phylogenetic and conserved motif analyses revealed sequence and structural conservation of the IR75 receptors across five noctuid species. Transcriptome data analysis further showed that IR75q.2 had the highest expression in the antennae among the clade, suggesting its importance in olfaction. Functional assays using the Xenopus oocyte expression system showed that IR75q.2 from two Spodoptera species displayed conserved ligand response profile, responding to three medium-chain fatty acids and their corresponding aldehydes, with nonanoic acid as the strongest ligand. Molecular docking and site-directed mutagenesis further identified Phe284/285 and Arg289/290 as conserved key residues involved in nonanoic acid recognition by IR75q.2. Finally, nonanoic acid was determined to elicit both electrophysiological and behavioral responses in adults of the two Spodoptera species. Taken together, our study reveals conserved ligand recognition profiles and key binding residues of IR75q.2 in two Spodoptera species, providing insights into the functional evolution of IR75 clade in the olfactory system.