Method for selecting exposure levels for the Drosophila sex-linked recessive lethal assay

The use of the Drosophila sex-linked recessive lethal assay for detecting mutagenicity of chemicals is well established. When compounds are tested by feeding adult flies, the National Toxicology Program protocol specifies a 3-day feeding regimen at an exposure level that produces about 30% mortality. Uptake of the test compound is monitored by feeding behavior, amount of excretion, or abdomen size (Woodruff et al: Environ Mutat 7:677-702, 1985). An alternate method for determining uptake is to add radiolabeled sucrose to the feeding solution and then to determine the amount of radioactivity in the flies (Gollapudi et al: Mutat Res 144:13-17, 1985). We have found that the addition of radiolabeled sucrose underestimates consumption for feeding exposures longer than 24 hr because sucrose is metabolized and as much as 30% of the label is excreted, presumably as 14CO2 or 3H2O. Here we describe a method for determining uptake of chemicals by adding 14C-leucine to the feeding solution. The incorporation of 14C-leucine is essentially linear over the 3-day feeding period, which permits accurate estimates of food consumption. Use of this method demonstrates that lower exposure levels of a chemical that do not produce mortality actually results in higher consumption by the flies. The method is proposed as a prescreen to select the appropriate exposure level for the sex-linked recessive lethal assay.