# Genome-Wide Characterization of Four Gastropod Species Ionotropic Receptors Reveals Diet-Linked Evolutionary Patterns of Functional Divergence

## TL;DR

This study explores how the evolution of smell-related genes in snails is linked to their different diets, revealing how feeding habits shape sensory evolution.

## Contribution

The study provides the first genome-wide analysis of ionotropic receptors in four gastropod species, linking their functional divergence to diet.

## Key findings

- Carnivorous snails have a significantly expanded set of ionotropic receptor (IR) genes.
- IR25b, a key olfactory receptor, shows repeated duplication across all four species.
- Most IRs are under purifying selection, but some show relaxed constraints and tissue-specific expression.

## Abstract

Gastropods rely heavily on olfactory perception to search for food, recognize other individuals, and respond to environmental cues. Ionotropic Receptors (IRs), which evolved from Ionotropic Glutamate Receptors (iGluRs), play an important role in these smell-related processes. However, the evolutionary patterns and functional diversity of these receptors in gastropods remain poorly understood. In this study, we identified and analyzed IR and iGluR genes in four gastropod species with different feeding habits: plant-eating, mixed-feeding, and carnivorous species. We examined their gene numbers, structural features, evolutionary patterns, and expression profiles. Our results showed that the carnivorous species contained a markedly expanded set of IR genes. In addition, the IR25b, a core component widely involved in olfactory signaling, underwent repeated duplication in all four species. Most receptors shared highly conserved structural features and showed signs of evolutionary stability, while some displayed strong tissue-specific expression, indicating specialized roles. These findings suggest that IR diversification may be linked to variation in chemosensory traits among gastropod species with different feeding habits. This work provides a foundation for understanding the molecular basis of smell in gastropods and offers insights into how feeding ecology shapes sensory evolution.

Ionotropic receptors (IRs) are a divergent subfamily of ionotropic glutamate receptors (iGluRs) that detect olfactory and environmental cues, influencing behaviors such as foraging and adaptation. To explore the evolution of IRs in relation to feeding ecology, we identified IRs and iGluRs from the genomes of four gastropods with distinct diets: Pomacea canaliculata (9 IRs/18 iGluRs), Bellamya purificata (10/22), Cipangopaludina chinensis (11/23), and Babylonia areolata (22/41). IRs were markedly expanded in B. areolata, suggesting lineage-specific diversification. Phylogenetic analysis grouped IRs and iGluRs into three clades, with IRs clustered with GluD, supporting early functional divergence following gene duplication. In all species, IR25b showed tandem duplication and played a central role in protein–protein interaction (PPI) networks. Most IRs were acidic, whereas IR-A and IR-C subgroups were basic, suggesting functional specialization among subfamilies. Structural analysis showed that IRs share conserved domains and motifs across species. Most IRs experienced purifying selection, while P. canaliculata showed relaxed constraints, suggesting weaker functional limitation. Collinearity analysis identified conserved genes, such as BarIR-A.6 and BarIR-D.1, across species. qPCR confirmed tissue-specific expression of IRs in multiple organs. Together, these results reveal the molecular features and evolutionary patterns of IRs in gastropods, highlighting their potential roles in olfaction and dietary adaptation.