Mechanisms of urea tolerance in urea-adapted populations of Drosophila melanogaster

When behavioral avoidance cannot prevent an animal from being exposed to no vel environmental toxins, physiological mechanisms must cope with the toxin and its effects. We are investigating the basis of urea tolerance in popul ations of Drosophila melanogaster that have been selected to survive and de velop in food containing 300 mmol l(-1) urea. Previous research has demonst rated that the urea-selected larvae have lower levels of urea in their body than control larvae reared under the same conditions. The current series o f experiments focuses on three possible ways of reducing urea levels in the body: urea metabolism, increased urea excretion and decreased urea uptake from the environment. We tested for urea metabolism directly, by assaying f or activity of two urea-metabolizing enzymes, and indirectly, by looking fo r reduced urea content of their medium. To measure urea excretion rates in whole animals, we reared control and urea-selected larvae on urea-containin g food (urea food), switched them to normal food and monitored the loss of urea from their hemolymph. We measured urea uptake by rearing control and s elected larvae on normal food, switching them to urea food and monitoring t he rate of urea appearance in the hemolymph. We found no evidence for urea metabolism by either direct or indirect methods. Control larvae excreted ur ea at a higher rate than selected, probably because they contained more ure a than the selected larvae and thus had a greater gradient for urea loss. T he rate of urea uptake in selected larvae was 2 mmol l(-1) h(-1) slower tha n the rate in control larvae, a difference that could account for the measu red differences in body urea levels. Thus the selected larvae appear to hav e adapted to urea exposure primarily by decreasing the ability of urea to e nter their body in the first place. The mechanism responsible for this redu ction in uptake is uncertain.