Regulatory variants of glycogen synthase contribute to differential glycogen content between Crassostrea gigas and C. sikamea

In marine bivalves, glycogen functions as a key energy source, influencing essential physiological processes such as growth, development, gametogenesis, and the reproductive cycle. Glycogen synthase (GYS), the key enzyme regulating glycogen biosynthesis, shows expression patterns directly correlated with seasonal glycogen fluctuations. This study identified genetic variations within the transcriptional regulatory region of the GYS gene in populations of Crassostrea gigas and C. sikamea. Two key single nucleotide polymorphism (SNP) sites, SNP -792-G/T and SNP -60-T/C, were identified to be associated with glycogen content. The polymorphisms in the GYS gene promoter result in species-specific differences in transcriptional binding activity, with C. sikamea showing higher activity in core region compared to C. gigas. Polymorphisms in this highly conserved promoter region alter the affinity of transcription regulatory elements for hypoxia-inducible factor 1 alpha (HIF-1α) and octamer-binding transcription factor 1 (OCT-1), thereby influencing GYS gene expression levels. These changes were related with interspecific differences in glycogen content between C. sikamea and C. gigas. Experimental evidence confirmed the specific binding of transcription factors to SNP-mutated promoter motifs. These SNPs represent critical species-specific regulatory sites and may serve as valuable markers for selecting oyster strains with high glycogen content. Our findings demonstrate that SNPs within the transcription regulatory elements of the GYS gene alter the binding affinity for transcription factors OCT-1 and HIF-1α, thereby playing a critical role in energy metabolism in oysters. Our study offers novel insights into the transcriptional regulation of the GYS gene and its contribution to glycogen storage.