Natures wonders: insect brain metamorphosis (Introduction)

by David Turell , Wednesday, July 26, 2023, 18:37 (484 days ago) @ David Turell

A magnificent study:

https://www.quantamagazine.org/insect-brains-melt-and-rewire-during-metamorphosis-20230...

"Metamorphosis is not an exception in the animal kingdom; it’s almost a rule. More than 80% of the known animal species today, mainly insects, amphibians and marine invertebrates, undergo some form of metamorphosis or have complex, multistage life cycles.

"The process of metamorphosis presents many mysteries, but some of the most deeply puzzling ones center on the nervous system. At the center of this phenomenon is the brain, which must code for not one but multiple different identities. After all, the life of a flying, mate-seeking insect is very different from the life of a hungry caterpillar. For the past half-century, researchers have probed the question of how a network of neurons that encodes one identity — that of a hungry caterpillar or a murderous lacewing larva — shifts to encode an adult identity that encompasses a completely different set of behaviors and needs.

"Truman and his team have now learned how much metamorphosis reshuffles parts of the brain. In a recent study published in the journal eLife, they traced dozens of neurons in the brains of fruit flies going through metamorphosis. They found that,...adult insects likely can’t remember much of their larval life. Although many of the larval neurons in the study endured, the part of the insect brain that Truman’s group examined was dramatically rewired. That overhaul of neural connections mirrored a similarly dramatic shift in the behavior of the insects as they changed from crawling, hungry larvae to flying, mate-seeking adults.

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"The earliest insects 480 million years ago emerged from eggs looking much like smaller versions of their adult selves, or else they continued their “direct development” to get steadily closer to their adult form, just as grasshoppers, crickets and some other insects do today. Complete metamorphosis seems to have arisen in insects only around 350 million years ago, before the dinosaurs.

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“'The nervous system has never been able to change the way it makes neurons,” Truman said. That’s partly because the nervous system in all insects arises from an array of stem cells called neuroblasts that mature into neurons. That process is older than metamorphosis itself and not easily modified after a certain stage of development. So even as nearly all the other cells in the fruit fly’s larval body are eliminated, most of the original neurons are recycled to function anew in the adult."

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"The researchers zoned in on the mushroom body, a region of the brain critical for learning and memory in fruit fly larvae and adults.

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"Truman and his team found that when the larvae undergo metamorphosis, only seven of their 10 neural compartments are incorporated into the adult mushroom body. Within those seven, some neurons die, and some are remodeled to perform new adult functions. All the connections between the neurons in the mushroom body and their input and output neurons are dissolved. At this transformation stage, “it’s kind of this ultimate Buddhistic situation where you have no inputs, you have no outputs,” Gerber said. “It’s just me, myself and I.”

"The input and output neurons in the three larval compartments that don’t get incorporated into the adult mushroom body completely shed their old identities. They leave the mushroom body and integrate into new brain circuits elsewhere in the adult brain. “You wouldn’t know that they were the same neurons, except that we’ve been able to both genetically and anatomically follow them through,” Truman said.

"The researchers suggest that these relocating neurons are only temporary guests in the larval mushroom body, taking on necessary larval functions for a while but then returning to their ancestral tasks in the adult brain.

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"In addition to the remodeled larval neurons, many new neurons are born as the larva grows. These neurons are not used by the larva, but at metamorphosis they mature to become input and output neurons for nine new computational compartments that are adult specific.

The mushroom body in the larva looks very similar to the adult version, Thum said, but “the rewiring is really intense.” It’s as if the inputs and outputs of a computational machine all got disrupted but still somehow maintained their wireless functionality, Gerber said. “It’s almost as if you would deliberately unplug and replug” the machine.

"As a result, the adult brain’s mushroom body is “fundamentally … a completely new structure,” said K. VijayRaghavan, an emeritus professor and former director of India’s National Center for Biological Sciences who was the main editor of the paper and was not involved in the study. There is no anatomical indication that memories could have survived, he added.
(my bold)

"Researchers have been excited by this question of whether a larva’s memories can carry through to the adult insect, Williams said, but the answer hasn’t been clear-cut."
(my bold)

Comment: a master work of unmasking how metamorphosis affects the brain's neurons. The memory problem is solved if you accept God designed the DNA to handle all of it, including all instinctual behavior in the larvae and the adults.