Genetic Variation in Long-Term and Short-Term Physiological Changes in Daphnia magna During Acclimation to High Temperature

The aquatic zooplankton crustacean Daphnia magna must be able to tolerate thermal stress in order to survive their native shallow ponds that are susceptible to drastic seasonal and diurnal temperature fluctuations as well as to globally increasing temperatures. Survival in such variable environments requires plastic responses that must include fundamental aspects of Daphnia biochemistry and physiology. Adaptive response to selection favoring such plastic phenotypes requires the presence of genetic variation for plastic response in natural populations. Adverse effects of elevated temperature on aquatic organisms are diverse and so are their plastic responses; among the most severe challenges aquatic organisms face when exposed to heat is the elevated oxidative stress. In this work we focused on short-term and longterm responses of Daphnia to temperature changes that increase its resistance to oxidative stress. Daphnia acclimated to stressful but non-lethal temperature (28oC) show longer survive during exposure to a lethal temperature (37oC) than those acclimated to the optimal temperature (18oC). Shortterm reciprocal switches between 18oC and 28oC result in intermediate temperature tolerance. These changes are accompanied by mirroring changes in total antioxidant capacity indicating the increased antioxidant capacity as a possible causative mechanism for heat tolerance gained from acclimation. The analysis of 6 geographically distinct genotypes representing a range of temperature tolerance levels shows a genetic difference in response to short-term and long-term acclimation as well as in the effect of antioxidant capacity on temperature tolerance. These results indicate a significant degree of local adaptation in heat and oxidative stress defenses in Daphnia and provide a better understanding of adaptive responses of this zooplankton crustacean to rising temperatures.