Communicating the nutritional value of sugar in Drosophila.

Sweet-insensitive Drosophila mutants are unable to readily identify sugar. In presence of wild-type (WT) flies, however, these mutant flies demonstrated a marked increase in their preference for nutritive sugar. Real-time recordings of starved WT flies revealed that these flies discharge a drop from their gut end after consuming nutritive sugars, but not nonnutritive sugars. We proposed that the drop may contain a molecule(s) named calorie-induced secreted factor (CIF), which serves as a signal to inform other flies about its nutritional value. Consistent with this, we observed a robust preference of flies for nutritive sugar containing CIF over nutritive sugar without CIF. Feeding appears to be a prerequisite for the release of CIF, given that fed flies did not produce it. Additionally, correlation analyses and pharmacological approaches suggest that the nutritional value, rather than the taste, of the consumed sugar correlates strongly with the amount (or intensity) of the released CIF. We observed that the release of this attractant signal requires the consumption of macronutrients, specifically nutritive sugars and l-enantiomer essential amino acids (l-eAAs), but it is negligibly released when flies are fed nonnutritive sugars, unnatural d-enantiomer essential amino acids (d-eAAs), fatty acids, alcohol, or salts. Finally, CIF (i) is not detected by the olfactory system, (ii) is not influenced by the sex of the fly, and (iii) is not limited to one species of Drosophila.