Divergent evolution of ionotropic receptor subtypes in a terrestrial isopod Armadillidium vulgare

Chemosensory systems in arthropods underlie diverse behaviors such as foraging, environmental recognition, and predator avoidance, and elucidating their molecular basis is a central issue in evolutionary biology. In insects, ionotropic receptors (IRs) include the coreceptors IR25a and IR8a, which are broadly conserved, and subtype receptors such as IR76b that together form response systems to diverse chemical stimuli. However, the evolutionary patterns of IR gene families in crustaceans, particularly terrestrial isopods, remain poorly understood. In this study, we performed identification of IR genes and phylogenetic analyses using publicly available genome data of the terrestrial isopod Armadillidium vulgare. Maximum likelihood phylogenetic analysis revealed clear orthologs of IR25a and IR8a, both forming well-supported clades with other arthropod sequences (bootstrap ≥99). In contrast, no clear ortholog of IR76b was identified within the canonical insect IR76b clade. Instead, one sequence (Avul 7642106.1) was recovered as a sister group to the insect IR76b clade and was therefore classified as an IR76b-like receptor. These results indicate an asymmetric evolutionary pattern in which co-receptors are conserved under strong purifying selection, whereas subtype receptors undergo lineagespecific diversification. Our findings provide a molecular basis for understanding the adaptive evolution of chemosensory systems associated with terrestrialization in crustaceans.