Genome-Wide Identification of 5-HT Receptor Gene Family in Razor Clam Sinonovacula constricta and Their Circadian Rhythm Expression Analysis

SIMPLE SUMMARY: Serotonin (5-HT) is a highly effective oxytocin widely used in aquaculture. However, the role of 5-HT in mollusks remains unclear. Limited research has been conducted on the structures of 5-HT receptor genes, particularly those associated with ovulatory function. In this study, 5-HT receptors were screened and characterized in the whole genome of Sinonovacula constricta. Some 5-HT-binding receptors had been tentatively identified, and their expression pattern exhibited a circadian rhythm. The findings will contribute to a deeper comprehension of the biological role of 5-HT in bivalve ovulation induction and enhance the investigation into its molecular mechanism. ABSTRACT: Serotonin (5-HT) is primarily distributed in the gastrointestinal and central nervous systems, where it plays a crucial role in regulating various physiological functions such as digestion, reproduction and establishing animal emotions. 5-HT is an effective oxytocin widely used in molluscan aquaculture, and its physiological functions are performed by binding to corresponding 5-HT receptors (5-HTRs). In this study, seven 5-HTR genes of Sinonovacula constricta (Sc5-HTRs) were identified and analyzed, and they were designated as Sc5-HT1A, Sc5-HT1D, Sc5-HT2-1, Sc5-HT2-2, Sc5-HT2-3, Sc5-HT4 and Sc5-HT6. Phylogenetic analysis showed that the seven Sc5-HTRs were conserved among mollusks, and the Sc5-HTRs were all transmembrane proteins. The seven Sc5-HTR genes were distributed on chromosome 1, 2, 13 and 14. After injecting 5-HT, there was a significant increase in mRNA expression levels of Sc5-HT1A (p < 0.05) and Sc5-HT2-3 (p < 0.01), while Sc5-HT4 decreased significantly (p < 0.01) compared to control groups which might be effective 5-HT receptors. Furthermore, two of the receptors (Sc5-HT2-3 and Sc5-HT4) were expressed in the circadian rhythm patterns, indicating their potential influence on the nocturnal spawning of S. constricta. Overall, these findings provide a theoretical basis for understanding the structures and functions of 5-HTR gene family members, and may facilitate the artificial propagation of mollusks.