Drosophila Learn Opposing Components of a Compound Food Stimulus

Dopaminergic neurons provide value signals in mammals and insects [1–3]. During Drosophila olfactory learning, distinct subsets of dopaminergic neurons appear to assign either positive or negative value to odor representations in mushroom body neurons [4–9]. However, it is not known how flies evaluate substances that have mixed valence. Here we show that flies form short-lived aversive olfactory memories when trained with odors and sugars that are contaminated with the common insect repellent DEET. This DEET-aversive learning required the MB-MP1 dopaminergic neurons that are also required for shock learning [7]. Moreover, differential conditioning with DEET versus shock suggests that formation of these distinct aversive olfactory memories relies on a common negatively reinforcing dopaminergic mechanism. Surprisingly, as time passed after training, the behavior of DEET-sugar-trained flies reversed from conditioned odor avoidance into odor approach. In addition, flies that were compromised for reward learning exhibited a more robust and longer-lived aversive-DEET memory. These data demonstrate that flies independently process the DEET and sugar components to form parallel aversive and appetitive olfactory memories, with distinct kinetics, that compete to guide learned behavior. •Flies trained with unpalatable sugar learn the sweet-, nutrient, and bad-taste qualities•Distinct dopamine neurons reinforce the positive and negative memories in parallel•Early conditioned aversion switches to longer-lasting nutrient-memory-guided attraction•Flies remember individual qualities of a complex food source Das et al. show that flies trained with unpalatable sugar learn both the nutritional and bad-taste qualities. Opposing memories are reinforced in parallel by distinct dopaminergic neurons. Conditioned behavior is initially aversive but soon switches to approach led by the longer-lasting nutrient-reinforced appetitive memory.