Brain Reconstruction During Metamorphosis: Why Do Larval Memories Not Persist in Adult Insects?

Metamorphosis remains one of the most fascinating enigmas of nature as it shows the dramatic physical change from one life stage to another. Over 80% of known animal species today undergo metamorphosis or possess complex, multistage life cycles.

Remodeling the Insect's Mind

Among the many mysteries surrounding metamorphosis, the most puzzling one involves the nervous system. At the center of it is the brain, which must code multiple identities.

In a study led by Professor James Truman from the University of Washington, scientists investigated metamorphosis's role in reshuffling brain parts. They traced neurons in the brains of fruit flies that undergo metamorphosis.

The research team had to trace individual cells and circuits to understand what is happening to the brain. Fortunately, the nervous system of a fruit fly provides a practical opportunity to do that. Although most of its body cells die while transforming into adulthood, many neurons in the brain do not.

To map the changes in the brain, Truman and his colleagues examined the genetically engineered fruit fly larvae containing specific neurons that shone a fluorescent green under the microscope. They discovered that this fluorescence often fades during metamorphosis, so they used a genetic method to turn on a red fluorescence in the same neurons by giving the insects a particular drug.

The team zoned in on the mushroom body, an area of the brain critical in learning and memory in both the larval and adult fruit flies. A bunch of neurons with long axonal tails communicate with the rest of the brain through input and output neurons that weave in and out of the string. This creates a network of connections that allows the insects to associate scent with good or bad experiences.

It was revealed that when the larvae undergo metamorphosis, only 7 of their ten neural compartments are incorporated into their adult mushroom bodies. The input and output neurons in the remaining three larval compartments shed their old identities entirely since they fail to enter the adult stage. After leaving the mushroom, they integrate into new brain circuits elsewhere in the adult brain.

Aside from the remodeled larval neurons, many new neurons are also born as the larva grows. Although the larva does not use these neurons, they mature during metamorphosis to become input and output neurons for nine new computational compartments specific to adult insects.

Fragility of Insect Memory

The type of memories living in the mushroom body of a fruit fly are associative memories, which typically involve smells guiding them toward or away from something. In addition, it was found that adult insects possibly cannot remember much of their larval life.

Aside from detailing the maturity of a larval brain to an adult brain, the research also provides clues as to how evolution made insect development take a wild detour from hungry larvae to mate-seeking adults. It also offers opportunities to compare nervous system effects in animals undergoing metamorphosis and those that do not.

The patterns of reorganization elsewhere in the brain are still unknown. However, some aspects of the mental capacities of the fruit flies and their responses to the world are likely shaped by their larval life.