Selection for longevity confers resistance to low-temperature stress in Drosophila melanogaster

One theory of the evolution of longevity says that improvement in life span is dependent on an increased ability to resist environmental stresses of all kind. Selective breeding of Drosophila melanogaster populations for longevity has demonstrably increased life span and also altered a number of other traits, such as resistance to starvation, desiccation, and ethanol fumes, and the ability to sustain longer flight. While the exact physiologic basis of some of these traits is not yet fully understood, at least some are known to derive from the properties of metabolic substrates of glycolysis. Improvement in those characters can depend partially, therefore, on altered stores of metabolites created from glycogen. Based on the known general relationship of some traits and the suspected basis in metabolism of others, we examine the possibility here that increased life span is accompanied by other traits that also confer physiologic resistance to stress. Specifically, we test the prediction that long-lived populations of fruit flies should be more resistant to low (prefreezing) and freezing temperature extremes. Both selected and control populations were found to be susceptible to prefreezing (1.5 degrees C) and freezing temperatures (0 degree C) here, but adults and pupae of the long-lived populations generally survived better in both situations, and at all durations of exposure. The resistance of individuals improved with acclimatization, but was superior in the long-lived populations whether thermal decline was rapid or stepwise. Cold resistant, long-lived populations also had significantly higher in vitro levels of glycerol, a cryoprotectant metabolite produced from glycogen. However, while adults and pupae of long-lived stocks were more resistant to cold, larvae of those stocks were more sensitive and survived relatively poorly at every length of exposure and acclimation. This surprising result implies that larvae maintain lower levels of cryoprotectant substances. Upon becoming pupae, however, stage-specific capabilities for environmental resistance and long life emerge. This conclusion agrees with a prior study of these stocks indicating that the uptake and use of nutrients in developing larvae are restricted in long-lived populations.