No evidence for aversive associative olfactory memory through metamorphosis in Drosophila

Do memories acquired through learning by insects in the larval stage persist throughout metamorphosis into adulthood? There are contradictory reports to answer this question. After a larval stage, holometabolous insects undergo pupation and metamorphosis, during which the body is substantially transformed. This remodeling also affects the brain: neuronal connections degrade, axons and dendrites prune and regrow, new neurons are generated, and new synaptic connections are established. Such drastic remodeling appears incompatible with reports from several insect species, including Drosophila melanogaster, suggesting that larval memories are maintained throughout metamorphosis and can be observed in the adult imago. We use aversive associative olfactory conditioning of wild-type and pruning-defective D. melanogaster larvae to revisit this long-standing and unresolved question. In contrast to previous reports, we found no evidence that memories acquired through classical aversive conditioning in the larval stage survive the neuronal remodeling processes during metamorphosis. Even when pruning and regrowth of larval Kenyon cells of the mushroom body, a central brain structure essential for associative learning, are largely prevented by genetic intervention, no maintenance of larval memories could be detected in the adult fly. We conclude that at least larval olfactory memories acquired through aversive classical conditioning are erased over the course of metamorphosis.