Consumption of bitter alkaloids in Drosophila melanogaster in multiple-choice test conditions

Drosophila melanogaster adapt their food consumption to their internal needs and avoid ingesting noxious molecules. Defects in the genes involved in these decisions induce behavioral alterations that are usually screened by monitoring flies feeding in 2-choice or in no-choice situations. Here, we introduce a new behavioral test in which groups of flies are given access to 6 capillary feeders (MultiCAFE) containing fructose mixed with a serial dilution of a test substance. Using quinine, we first showed that fly density, distance between capillaries, and order of presentation have a minor impact on the discrimination performances of the flies. Fly discrimination was also only marginally affected by the type of test (no-choice, binary, or multiple-choice). Interestingly, the feeding reduction was well correlated with a reduction of the firing elicited by the mixture in sugar-sensitive gustatory receptor neurons, suggesting that several mechanisms concur to allow flies to make their choices. In addition to quinine, flies exhibited marked dose-dependent aversions to the consumption of berberine, caffeine, lobeline, nicotine, papaverine, strychnine, and theophylline, which all taste bitter to humans. Thus, despite of the multiplicity of choices available, flies consistently avoid alkaloids mixed with a sugar solution, and their choices are strongly dependent on their taste system. The MultiCAFE assay represents an interesting alternative to other feeding tests, in that it allows monitoring of the absolute consumption while also requiring less flies and time to run than other assays.