EVOLUTION AND THERMAL RESPONSIVENESS OF CHEMOSENSORY RECEPTORS IN PACIFIC ABALONE

Marine mollusks are ectothermic organisms whose physiology and survival are tightly constrained by environmental temperature. Although transient receptor potential (TRP) channels are well-established thermosensors, recent evidence suggests that additional thermosensitive receptors are embedded within chemosensory gene families, including G protein–coupled receptors (GPCRs) and ionotropic receptors (IRs). However, the diversity, evolution, and thermal responsiveness of these receptor families in marine mollusks remain poorly understood. Here, we integrated comparative genomics, transcriptomics, phylogenetic analyses, and molecular validation to systematically characterize putative thermosensitive chemosensory receptors across six mollusk species: Haliotis discus hannai, Lottia gigantea, Mizuhopecten yessoensis, Magallana gigas, Aplysia californica, and Octopus bimaculoides. Homology-based analyses identified 1,338 class-A GPCRs and 137 IR-related genes. Phylogenetic reconstruction revealed extensive lineage-specific expansion, including 348 mollusk-specific GPCRs and 85 mollusk-specific IRs, highlighting dynamic evolutionary diversification of these sensory receptor families. Focusing on Pacific abalone (H. discus hannai), we examined tissue-specific and temperature-responsive expression patterns under acute cold and heat stress using transcriptome profiling and quantitative PCR. Mollusk-specific GPCRs and IRs were predominantly expressed in sensory-associated tissues, particularly eyes and tentacles, with minimal expression in non-sensory tissues such as gills and hepatopancreas. Notably, 14 GPCRs and 5 IRs were significantly upregulated under both thermal extremes, suggesting conserved transcriptional responses to temperature stress. Together, these findings provide a comprehensive overview of chemosensory receptor evolution in mollusks and identify candidate thermoresponsive GPCRs and IRs that may contribute to temperature perception and thermal adaptation in marine ectotherms under ongoing climate change.