System-Like Consolidation of Olfactory Memories in Drosophila

System consolidation, as opposed to cellular consolidation, is defined as the relatively slow process of reorganizing the brain circuits that maintain long-term memory. This concept is founded in part on observations made in mammals that recently formed memories become progressively independent of brain regions initially involved in their acquisition and retrieval and dependent on other brain regions for their long-term storage. Here we present evidence that olfactory appetitive and aversive memories in Drosophila evolve using a system-like consolidation process. We show that all three classes of mushroom body neurons (MBNs) are involved in the retrieval of short- and intermediate-term memory. With the passage of time, memory retrieval becomes independent of alpha'/beta' and gamma MBNs, and long-term memory becomes completely dependent on alpha/beta MBNs. This shift in neuronal dependency for behavioral performance is paralleled by shifts in the activity of the relevant neurons during the retrieval of short- term versus long-term memories. Moreover, transient neuron inactivation experiments using flies trained to have both early and remote memories showed that the alpha'/beta' and gamma MBNs have a time-limited role in memory processing. These results argue that system consolidation is not a unique feature of the mammalian brain and memory systems, but rather a general and conserved feature of how different temporal memories are encoded from relatively simple to complex brains.