Animals often decide between alternative actions according to their current needs, and hence the value they assign to each of the competing options. This process is of special relevance during nutrient balancing, in which animals choose between different food sources according to their current nutritional state. How such value-based decision making is implemented at the molecular and neuronal level in the brain is not well understood. Here we describe Drosophila melanogaster food choice as a genetically tractable model to study value-based decision making in the context of nutrient balancing. When faced with a choice between yeast and an alternative food source, flies deprived of protein prefer the yeast. We show here that mating status is a critical modulator of this decision-making process in females and that it relies on the action of the sex peptide receptor in internal ppk(+) sensory neurons. Neuronal TOR/S6K function is another critical input to this decision, possibly signaling the fly's current nutritional status. We propose that the brain uses these internal states to assign value to external sensory information from potential food sources, thereby guiding food choice and ensuring nutrient homeostasis.
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