Expression regulation of Fad and Mboat gene families to cold stress in the noble scallop Chlamys nobilis: A comparative analysis across bivalves.

Cold waves exacerbated by global warming and the resultant sharp drops in coastal water temperature pose serious threats to the sustainability of bivalve aquaculture. The fatty acid desaturase (Fad) and membrane-bound O-acyltransferase (Mboat) gene families play essential roles in maintaining membrane lipid homeostasis and facilitating cold adaptation. Although Fad genes have been investigated in several bivalves, the role of Mboat genes in cold stress and how these two families coordinately regulate lipid remodeling under cold stress remain largely unclear. This study performed a comprehensive genome-wide identification of Fad and Mboat genes across 35 bivalve species, identifying 3-11 Fad and 6-11 Mboat members per species, classified into four (Fads, Fads6, Degs, Scd) and six (Lpcat, Mboat7, Porcn, Hhat, Dgat, Soat) subfamilies, respectively. Phylogenetic analysis revealed a significant expansion of the Fads subfamily in Mytilus and Venerida, lineage-specific expansion of Scd in Ostreida, and expansion of the Lpcat subfamily in Pectinida, Adapedonta, and Cardiida, suggesting adaptive evolution of lipid metabolic pathways in bivalves. Focusing on the noble scallop Chlamys nobilis, tissue-specific expression profiles were characterized, with CnScd1 highly enriched in blood and kidney and CnScd2 almost exclusively expressed in the female gonad. To investigate the response of these genes to cold stress, C. nobilis were exposed to acute and chronic cold stress. The results revealed complex transcriptional response patterns of the Fad and Mboat families to cold stress in bivalves, and identified multiple candidate genes showing consistent responses under both acute and chronic stress regimes, including CnScd1, which was significantly upregulated by 20-fold under both conditions, as well as CnDegs1 and CnLpcat1/2/3, which were steadily induced during chronic cold exposure. Furthermore, several CnMboat genes exhibited an inverted U-shaped expression peak around 10 °C, suggesting that this temperature may represent a critical physiological acclimation window. These results suggest that the Fad and Mboat families may be involved in membrane lipid adaptation, although the underlying mechanisms and functional outcomes still require further validation. This study provides new insights into the molecular mechanisms of cold adaptation in marine bivalves, and the above genes may serve as candidate markers for cold tolerance breeding, although further functional validation and population genetic assessments are still needed. Meanwhile, these findings also provide a theoretical foundation for molecular marker-assisted breeding of cold-tolerant varieties and the sustainable development of bivalve aquaculture.