Evolutionary compromises in ecological adaptation: urea and ammonia tolerance in Drosophila suzukii and Drosophila melanogaster

The invasive species Drosophila suzukii has evolved morphological and behavioral adaptations to lay eggs under the skin of fresh fruits. This results in severe damage of a wide range of small fruits, making this species a serious agricultural and economical threat. Drosophila suzukii females typically lay few eggs per fruit, preferring not infested fruits. Hence, larvae are exposed to a reduced amount of nitrogenous waste. Differently, the innocuous Drosophila melanogaster lays eggs on fermented fruits already infested by conspecifics, with larvae developing in a crowded environment with accumulation of nitrogenous waste such as ammonia and urea. The observed differences in oviposition site and larval ecological niche suggest that these species might differ in behavioral and physiological mechanisms used to cope with nitrogenous waste. We investigated how different concentrations of ammonia and urea affect fecundity and larval development in both species. Females and larvae of D. suzukii showed greater sensitivity to high concentration of both compounds, with a dramatic decrease in fecundity and egg viability. To better understand the pathways underlying these differences, we evaluated the effect on ornithine aminotransferase and glutathione-S-transferase, two enzymes involved in nitrogen metabolism and stress response that are expressed during larval development. Both ammonia and urea significantly reduced the expression of these enzymes in D. suzukii compared to D. melanogaster. This manifests how the ecological shift of D. suzukii to fresh fruit resulted in less efficient detoxifying and excretory mechanisms, with important implications for evolutionary biology and applied research.