Characterization and Functional Analysis of Fads Reveals Δ5 Desaturation Activity during Long-Chain Polyunsaturated Fatty Acid Biosynthesis in Dwarf Surf Clam Mulinia lateralis

Fatty acid desaturases (Fads), as key enzymes in the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs), catalyze the desaturation between defined carbons of fatty acyl chains and control the degree of unsaturation of fatty acids. In the present study, two Fads genes, designatedMulFadsAandMulFadsB, were identified from the genome of the dwarf surf clamMulinia lateralis(Mollusca, Mactridae), and their spatiotemporal expression was examined. MulFadsA and MulFadsB contained the corresponding conserved functional domains and clustered closely with their respective orthologs from other mollusks. Both genes were expressed in the developmental stages and all tested adult tissues ofM. lateralis, withMulFadsAexhibiting significantly higher expression levels in adult tissues thanMulFadsB. Subsequently, the effects of dietary microalgae onFadsexpressions in the dwarf surf clam were investigated by feeding clams with two types of unialgal diets varying in fatty acid content, i.e.,Chlorella pyrenoidosa(Cp) andPlatymonas helgolandica(Ph). The results show that the expressions ofMulFadswere significantly upregulated among adult tissues in the Cp group compared with those in the Ph group. In addition, we observed the desaturation activity of MulFadsA via heterologous expression in yeasts, revealing Δ5 desaturation activity toward PUFA substrates. Taken together, these results provide a novel perspective onM. lateralisLC-PUFA biosynthesis, expanding our understanding of fatty acid synthesis in marine mollusks.