Fatty acid profiles and tolerance to temperature extremes in <i>Daphnia pulex</i>

Identification of physiological processes setting thermal tolerance limits is essential to describing adaptive response to temperature changes. We used the North American Daphnia pulex complex, which makes a remarkable model for comparative physiology as it is composed of clones differing in heat tolerance and ploidies, and with a wide geographic distribution. The fatty acid composition of 18 diploid and triploid D. pulex clones acclimated to 16°C and 24°C was measured and compared with their tolerance to extreme high and low temperatures (CTmax and CTmin, respectively). Eicosapentaenoic acid (EPA) relative content showed a strong negative relationship with CTmax and a clear association with CTmin. Higher unsaturation and peroxidation indices were associated with better cold tolerance, whereas saturated fatty acids and monounsaturated fatty acids were associated with lower cold tolerance. Triploid D. pulex clones accumulated more EPA and had lower CTmin than diploid clones (better cold tolerance). Triploid clones retained more omega-6 polyunsaturated fatty acids at high temperature. CTmax was positively correlated with CTmin, suggesting the existence of important constraints in temperature tolerance caused by fatty acid composition.