Reorganization of membrane lipids during fast and slow cold hardening in Drosophila melanogaster

Rapid cold hardening is a naturally occurring phenomenon in insects that is thought to be responsible for increased cold tolerance during diurnal variations in temperature. The underlying physiological mechanisms are still not fully resolved but, in Drosophila melanogaster (Meigen 1830), rapid cold hardening is accompanied by specific changes in the membrane lipid composition. To further understand the link between rapid cold hardening and adjustments in the membrane lipid composition, the present study investigates how different rates of cooling affect thermotolerance and the composition of phospholipid fatty acids. Female Drosophila are cooled gradually from 25 to 0 degrees C at 0.01, 0.05, 0.1 or 0.5 degrees C min(-1), respectively, and, subsequently, phospholipid fatty acid composition and survival after a 1-h cold shock at -5 degrees C is measured. The rapid cold hardening treatments all influence cold tolerance differently so that short and intermediate rapid cold hardening treatments (0.05, 0.1 or 0.5 degrees C min(-1) cooling rates) increase cold shock survival, whereas the slow cooling treatment (0.01 degrees C min(-1)) decreases survival relative to an untreated control. The intermediate rapid cold hardening treatments (0.05 or 0.1 degrees C min(-1)) induce a similar type of response characterized by an increase in the molar percentage of linoleic acid, 18:2(n-6), at the expense of 16:0 and 18:1(n-9), which leads to an increase in the degree of unsaturation. The slowest cooling treatment (0.01 degrees C min(-1)) results in a large increase in cis-16:1(n-7) and significant reductions in the saturated phospholipid fatty acids 16:0, 18:0 and the unsaturated 16:1(n-9) and 18:2(n-6) fatty acids. These changes cause a slight decrease in the average length of the phospholipid fatty acids and an increase in the overall ratio of unsaturated vs. saturated fatty acids. These findings demonstrate that the rate of cooling is important for both the reorganization of membrane lipids, and for the degree of acquired cold tolerance during rapid cold hardening, and they suggest an important role for rapid cold hardening during diurnal rather than seasonal temperature changes.