Molecular Cloning and Functional Characterization of Δ9 Fatty Acyl Desaturase in Scylla paramamosain: Insights into Its Regulatory Roles Under Salinity Challenge, Dietary Fatty Acid Alteration and Starvation Stress

Δ9 fatty acyl desaturase (Δ9 FAD) exerts important regulatory effects on fatty acid metabolism and membrane fluidity. However, its existence and the effects of nutritional regulation and ambient salinity on its expression are currently unknown in mud crab (Scylla paramamosain). In this study, the full cDNA sequence of Δ9 fad was isolated from the hepatopancreas of mud crab, which consists of 1468 bp and encodes a polypeptide of 347 amino acids. Sequence analysis revealed that the protein possessed typical characteristics of Δ9 FAD including three histidine-rich domains (HRLWSH, HRVHH and HNYHH), four transmembrane regions and three conserved motifs (FFSHMGW, QRKYY and PWDY). The results of tissue distribution indicated that the mud crab’s Δ9 fad expression was enriched in the hepatopancreas, with relatively lower levels found in the thoracic ganglia and cranial ganglia. Δ9 fad expression in the hepatopancreas was significantly influenced by dietary fatty acid profiles, and the replacement of fish oil with soybean oil markedly downregulated the transcriptional levels of hepatopancreatic Δ9 fad. Furthermore, the transcriptional levels of Δ9 fad were also significantly affected by ambient salinity. Under chronic salinity stress, the 7‰, 12‰, 17‰ and 22‰ salinity groups showed higher transcriptional levels of Δ9 fad in the hepatopancreas than the 27‰ salinity group, and the 7‰ salinity group significantly differed from the 27‰ salinity group. Under the acute salinity stress condition, the Δ9 fad transcriptional levels of muscle in the 14‰ and 4‰ salinity groups were higher than those in the 24‰ salinity group at 6 h and 12 h. By contrast, Δ9 fad in the hepatopancreas exhibited a slow response in reaction to low salinity stress, and only the 4‰ salinity group at 96 h had a higher value than the 24‰ salinity group. In addition, as for the starvation stress experiment, Δ9 fad expression in the starvation group was dramatically reduced when compared with the feeding group. These findings improve our understanding of the physiological roles of Δ9 fad and its regulatory mechanisms in crustaceans.