Endogenous LC-PUFA biosynthetic capability in dwarf surf clam Mulinia lateralis: characterization and functional analysis of fatty acyl elongases and a desaturase

Elongases of very long-chain fatty acids (Elovls) and fatty acyl desaturases (Fads) are two key enzymes in the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs). Here, we identified and characterized five Elovl genes ( MulElovl2/5 , MulElovl4 , MulElovlc , MulElovl7a , and MulElovl7b ) from the genome of Mulinia lateralis , a potential model bivalve. Expression analysis revealed that MulElovl2/5 , MulElovl4 and MulElovlc were highly expressed during the middle stage of embryonic development, whereas MulElovl7a and MulElovl7b peaked in mid-to-late development before declining. Tissue-specific expression showed that MulElovl2/5 and MulElovl4 were predominantly expressed in the digestive gland. Additionally, we examined the effects of dietary LC-PUFA composition on MulElovls expression by feeding M. lateralis two unialgal diets: Chlorella pyrenoidosa (Cp), rich in linoleic acid (LA, C18:2n-6) and α-linolenic acid (ALA, C18:3n-3), and Platymonas helgolandica (Ph), rich in eicosapentaenoic acid (EPA, C20:5n-3). Notably, MulElovls were significantly upregulated in most tissues of the Cp group, suggesting a compensatory mechanism under LC-PUFA-deficient conditions. Functional assays revealed distinct substrate specificities: MulElovl2/5 elongated C18 and C20 PUFAs, while MulElovl4 and MulElovlc efficiently converted C20 to C22 PUFAs. Additionally, MulFadsB exhibited Δ6 desaturase activity, enabling the conversion of C18 PUFAs. These findings demonstrate that M. lateralis possesses endogenous biosynthetic pathways for arachidonic acid (ARA) from LA via Δ6 desaturation, elongation, and Δ5 desaturation, as well as partial elongation of EPA to C22:5n-3. Our findings provide new insights into a comparison of endogenous LC-PUFAs synthetic capabilities in mollusks and highlight M. lateralis as a valuable model for investigating lipid metabolism in marine bivalves.