Evolution of ammonia and urea tolerance in Drosophila melanogaster: resistance and cross-tolerance.

We examined whether populations of Drosophila melanogaster could evolve a genetically based tolerance to high levels of toxic compounds (urea or ammonia) added to their larval food medium. We also examined whether tolerance to one compound may impart cross-tolerance to other compounds. Five populations selected for ammonia tolerance (AX), five populations selected for urea tolerance (UX), and five unselected controls (AUC) were assayed for developmental time, viability, and female fertility. These characteristics were measured on each of the 15 populations reared on one of three larval food conditions (plain banana-molasses, 0.35 M NH(4)Cl, or 0.266 M urea). On urea-supplemented media, the urea-selected populations developed fastest and expressed the highest viability; the ammonia-selected populations developed significantly faster and had a higher viability than the controls. Similarly, on ammonia-supplemented media, the ammonia-selected populations developed fastest and expressed the highest viability; the urea-selected populations developed significantly faster and had a higher viability than the controls. This suggests that a cross-tolerance exists for resisting different toxic compounds. Urea-selected females reared on urea-containing food media displayed superior fecundity, without any observable cross-tolerance effect. When all populations were reared on food containing 0.266 M urea, the urea-selected populations had the lowest levels of urea in their tissues. All populations reared on food containing 0.37 M ammonia or 0.266 M urea, contained more ammonia in their tissues than did populations reared on plain food.