DOES SELECTION FOR STRESS RESISTANCE LOWER METABOLIC-RATE

Environmental stress reduces the fitness of organisms, and in this man ner may shape species distribution and evolutionary change. Ecologists have long been interested in physiological mechanisms that reduce the effects of stress and result in improved fitness. It has previously b een suggested (1) that selection for stress resistance will bring abou t the evolution of lower standard metabolic rates and (2) that organis ms with increased stress resistance will have a lower metabolic rate d uring stressful conditions, compared to stress-sensitive organisms. We have tested these hypotheses using replicate populations of Drosophil a melanogaster. Desiccation and starvation-selected populations and th eir controls, as well as populations selected for postponed senescence and their short-generation controls were examined. The metabolic rate of each selected and control population was measured under three diff erent environmental conditions: (1) desiccation (no food or water), (2 ) starvation (water present, no food), and (3) food condition (food an d water present). The carbohydrate and lipid content of each populatio n were also measured. All selected and control populations lowered the ir metabolic rates when measured under starving conditions compared to food conditions. The metabolic rates of flies under desiccating condi tions did not differ from food conditions regardless of selection trea tment. When comparing the mass-specific metabolic rates of stress-sele cted populations to their controls, stress-selected populations had a significantly lower mass-specific metabolic rate in all cases, regardl ess of the environmental conditions under which the measurements were made. However, once the mass of nonmetabolizing lipid and carbohydrate was accounted for and metabolic rate was expressed in carbohydrate-li pid free mass, selected populations did not differ significantly in me tabolic rate from control populations, regardless of the environmental condition under which the measurements were made. We conclude that in the case of D. melanogaster, resistance to stress does not require th e evolution of a lower metabolic rate. Instead, the accumulation of me tabolic resources such as carbohydrate and lipid may be important for resistance to specific forms of stress.