Evolutionary diversification and expressional profile of adrenergic receptors in the Pacific oyster Crassostrea gigas

Adrenergic receptors (ARs) specifically recognize and bind catecholamines via conserved seven-transmembrane G protein-coupled receptor (GPCR) domains, which are regarded as critical mediators of immune responses in molluscan species.

In the present study, six ARs were identified in the Pacific oyster Crassostrea gigas, which exhibited conserved structural features of G protein-coupled receptors (GPCRs that were characterized by seven transmembrane helices (TM1-TM7) and a DR(Y) motif within the third intracellular loop. These receptors were classified into two subfamilies: α-type (Cgα1A, Cgα2A, Cgα2C, Cgα2Da, and Cgα2Db-ARs) and β-type (Cgβ2-AR). Notably, evolutionary divergence between the Cgα2Da and Cgα2Db subtypes has led to the absence of their orthologs in most vertebrate species. Both subtypes (Cgα2Da and Cgα2Db) are under relaxed purifying selection (Ka/Ks = 0.644 and 0.828, respectively). Transcriptomic profiling revealed distinct spatiotemporal expression patterns, with Cgα2A-AR, Cgα2C-AR, and Cgα2Da-AR predominantly expressed in hemocytes. Among them, Cgα2C-AR and Cgα2Da-AR were enriched in granulocytes, whereas Cgα2A-AR was preferentially expressed in agranulocytes. Moreover, transcripts of Cgα2A-AR, Cgα2C-AR, and Cgβ2-AR in hemocytes increased significantly following the first Vibrio splendidus stimulation, with Cgα2A-AR exhibiting a significant increase again after the secondary stimulation.

Collectively, these results suggested that the retention of Cgα2Da/α2Db-AR subtypes in oysters under relaxed selection pressure reflects an evolutionary strategy for immune adaptability. Their lineage-restricted diversification drives differential expression (e.g., Cgα2Da-AR in granulocytes) and post-challenge resilience (e.g., Cgα2A-AR upregulation), balancing energy conservation and pathogen defense. These findings provided a foundation for further elucidation of their immunoregulatory roles in oysters and contribute to an improved understanding of AR evolution in invertebrates.