METABOLIC RESERVES AND EVOLVED STRESS RESISTANCE IN DROSOPHILA-MELANOGASTER

We have examined starvation and desiccation resistance in 43 outbred p opulations of Drosophila melanogaster that have diverged from a common ancestral population as a result of a variety of defined selection pr otocols. The populations differ up to 8.5-fold in desiccation resistan ce and up to 10-fold in starvation resistance. We used these populatio ns to search for evolved physiological changes that might explain the differences in stress resistance. We examined two hypotheses for incre ased stress resistance that had been proposed previously in the litera ture: (1) that increments in starvation resistance are principally the result of differential lipid accumulation, and (2) that changes in gl ycogen accumulation play a role in evolved increases in resistance to desiccation stress. By quantifying desiccation resistance, starvation resistance, lipid content, and carbohydrate content in each of our pop ulations of flies, we were able to demonstrate strong correlations bet ween the capacity of the flies to resist starvation and the quantity o f lipid or carbohydrate that the flies had stored, The strongest corre lation (R-2 = 0.99) was observed when the total energy content of both the lipid and carbohydrate stores was regressed against starvation re sistance. These results demonstrate that the flies responded to select ion for starvation resistance through a genetically determined increas e in both lipid and carbohydrate storage. Similar analyses of the corr elation between lipid storage or total energy storage and desiccation resistance revealed no significant correlations. Carbohydrate storage was significantly correlated with desiccation resistance in female but not in male flies. These results suggest that different forms of stre ss are resisted with distinct physiological mechanisms and that the ev olutionary response of the flies to stress selection is specific to th e stress imposed.