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

Abstract

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.