PIERCE, V.A.; MANGOGNA, J.; College of Staten Island/CUNY, NY: Urea Uptake Rates Respond to Changing Food Concentrations in Urea-Resistant and Susceptible Drosophila Populations

We are using laboratory-selected D. melanogaster populations to investigate physiological mechanisms of toxin resistance. Larvae from replicate fly populations selected for resistance to 300 mM urea take up urea more slowly during development than susceptible wild-type larvae. We have characterized the relationship between food urea concentration and initial urea uptake rates. We fed resistant and wild-type larvae foods containing 0, 200, 400, 650 and 900 mM urea and then assayed the urea concentration of hemolymph from all groups. Urea uptake rates in mM urea/hr were calculated from hemolymph urea concentrations and feeding time. Analysis of variance indicates urea uptake rates are significantly affected by the interaction between replicate number and food urea concentration (p<0.001), selection treatment (p<0.003), replicate number (p<0.001) and food urea concentration (p<0.001). We interpret the significant interaction term and main effect of replicate number as a block effect because replicate populations were assayed on different days. Increasing urea concentration in the food significantly increased the rate of uptake, as expected for a system where urea entry is likely to be diffusion-dependent. Graphically, rates appear to level off between 650 and 900 mM urea for both types of larvae, suggesting it may be a saturable process. The significant selection term indicates that resistant and wild-type larvae differ in their response to increasing urea concentrations. While the shape of the response curve is similar for the resistant and wild-type larvae, the uptake rates are substantially lower for the selected larvae. Thus the evolved mechanism of toxin resistance may involve quantitative rather than qualitative adaptations.