Abstract: 【Aim】 In low temperature environment, insects will activate the physiological regulation mechanism in vivo to stabilize their own metabolism, and fat metabolism plays an important role in the process of resisting low temperature in insects. This study aims to explore the changes in fat metabolism in Monochamus alternatus larvae under low temperature and its influence on the cold tolerance of M. alternatus. 【Methods】 The 4th instar larvae of M. alternatus reared at the room temperature (25℃) were cultured in a constant temperature incubator at 25℃ (control) and 4℃ (cold acclimation), respectively. After 7 d, the larvae were dissected and their fat bodies were collected, the changes of lipid droplets were observed, and the fat content in the fat body was measured. The composition and content of free fatty acids were detected by gas chromatography-mass spectrometry (GC-MS), and the transcript levels of genes of key enzymes involved in fatty acid β-oxidation ［carnitine palmitoyltransferase 1 (CPT1), 4-ketoacyl-CoA thiolase (4KCT), very long chain acyl-CoA dehydrogenase (VLCAD), enoyl-CoA hydratase (ECH) and 3-hydroxyacyl-CoA dehydrogenase (3HCD-1)］ were assayed by RT-qPCR. 【Results】 After the 4th instar larvae of M. alternatus were subjected to cold acclimation (4℃) for 7 d, lipid droplets in the fat body became smaller, the lipid droplet density declined and the fat content decreased as compared to those in the control. However, the composition of fatty acids did not change. The main fatty acids in the fat body of the 4th instar larvae of both the control group and cold acclimation group were C16∶0, C16∶1, C18∶0, C18∶1 and C18∶2, of which the relative content of C18∶2 in both the two groups was the highest, decreasing from 31.83%±8.82% to 25.16%±2.88% after cold acclimation. After the 4th instar larvae of M. alternatus were subjected to cold acclimation, the relative contents of C16∶0, C16∶1 and C18∶2 in the fat body decreased, while the relative contents of C18∶0 and C18∶1 increased. Among the five main fatty acids, the relative abundance of various fatty acids in the fat body of the 4th instar larvae in the cold acclimation group was reduced compared with that in the control, of which the relative abundance of C16∶0, C16∶1 and C18∶2 decreased significantly. However, the double-bond index of free fatty acids in the fat body of the 4th instar larvae in the cold acclimation group was increased by 3.88% as compared to that in the control. The expression level of VLCAD gene in the cold acclimation group was significantly up-regulated as compared with that in the control group. 【Conclusion】 In low temperature environment, M. alternatus larvae maintain basic metabolism by consuming fat, and the degradation level of fatty acids in the fat body increased. Unsaturated fatty acids play a key role in the cold resistance of M. alternatus. Regulation of lipid metabolism is an important survival strategy for M. alternatus to cope with low temperature.

Key words: Monochamus alternatus, low temperature, cold acclimation, fat, fatty acid, lipid metabolism, cold resistance