Regeneration sounds like a superpower. It is the ability to regrow lost tissues, organs, or limbs. Some creatures, like axolotls or starfish, do this effortlessly. Their bodies replace what was damaged as if nothing happened. Scientists have long studied how this magical process begins. But what about the ending? How does regeneration know when to stop?
Researchers at the University of Illinois explored this mystery. Their new study in Science Advances focused on a tiny but mighty animal: the fruit fly larva, Drosophila. This insect may seem unimpressive, but its genetic secrets are helping scientists unravel the rules of regeneration.
The Mystery of Regeneration’s Final Act
Rachel Smith-Bolton, an associate professor of cell and developmental biology, has spent years studying regeneration. She wondered not just how it begins, but how it ends.
Gene expression during regeneration. (CREDIT: Science Advances)
“Many people have asked the question, how does regeneration begin, and why does it begin in some animals and tissues and not in others,” she said. “Those are really important questions and things we’ve been looking at, but not a lot of people have asked, at the end of the process . . . how does it end and rebuild the structure it’s supposed to rebuild?”
Smith-Bolton and her team, including graduate student Anish Bose, examined the larval fruit fly’s imaginal discs. These discs are small groups of cells inside the larva that will become adult body parts like wings and legs. If these discs are damaged before the larva becomes a pupa, they can regrow and form normal structures.
This regeneration sounds simple, but there is an unseen problem. How does the body know it has grown enough? What stops the growth at the perfect time to avoid mistakes?
Zelda: The Gene That Closes the Chapter
The team investigated a gene with a playful name, Zelda. Don’t let the name fool you. Zelda plays serious roles in development. The scientists used a special version of the gene that switches off when exposed to blue light. They damaged the larval wing discs and turned Zelda off at different times to see what happened.
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In flies with Zelda switched off, normal wing discs developed fine, and damaged discs started to regrow. But the new cells lost their sense of direction. The regrown wings emerged with missing veins, misplaced bristles, and boundaries blurred between sections.
“One of the most exciting findings from the study is Zelda’s surprising specificity during regeneration,” said Bose, the first author of the study. “During the mid-stages of regeneration, Zelda becomes crucial for the wing disc to properly recover. This reveals a striking distinction between how tissues grow during development and how they repair themselves after injury.”
Pioneer Factors: The Gatekeepers of Genes
How does Zelda do this? Zelda produces a transcription factor, a protein that turns genes on or off. This protein belongs to a rare group called pioneer transcription factors. These special proteins unlock tightly shut parts of the genome. They open these regions so genes can activate and tell cells what to become.
“This class of transcription factor called a pioneer transcription factor. And what that means is that if a region of the genome is tightly closed down to not express a gene, it can go in and open that region up in order to allow expression of a gene,” Smith-Bolton explained. “Our hypothesis now is that pioneer transcription factors may play important roles in these shifts.”
Zld is important for cell fate and patterning during regeneration. (CREDIT: Science Advances)
Although Zelda is specific to arthropods, the way it works links to human health. Many cancers involve cells growing and changing without control. Studying pioneer transcription factors has helped identify cancer treatments. Understanding Zelda may also guide regenerative therapies for humans.
“If the idea behind regenerative therapy is that you’re going to add factors that drive the process, you have to know how to control those factors appropriately and also understand what mistakes they might be causing and how you can prevent those mistakes,” Smith-Bolton said. “How would you control and prevent changes in pattern and cell fate? As a lab, we’ve been identifying some of things that help prevent those errors.”
Bose is also curious about Zelda’s role beyond regeneration.
“Could artificially increasing Zelda levels during normal development be harmful? And what molecular mechanisms are in place to keep Zelda in check when regeneration isn’t occurring?” he asked. “These are the exciting directions our lab is eager to explore next.”
The Broader World of Regeneration
Regeneration is not the same for all animals. Axolotls can regrow limbs and even spinal cords. Mammals, including humans, can only partially regenerate tissues like liver, digit tips, or muscles. As we age, even these abilities decline. Limbs, joints, and heart tissues regenerate very poorly.
The fruit fly wing imaginal disc offers a window into this process. Damaged wing discs regrow through signals such as reactive oxygen species, JNK and p38 signaling, and WNT signaling. These pathways control cell growth and identity. During regeneration, cells often lose their identity markers. They must restore these at the right time to create a proper wing.
Zld binds near genes important for wing development and morphogenesis. (CREDIT:
The researchers used a clever system to study this. They inserted a GAL4 transgene into the rotund gene, which controls part of the wing disc. At cool temperatures, GAL4 is off. When the temperature rises, GAL4 activates a gene that kills cells in the wing disc. After damaging the tissue, the scientists shifted the temperature back down, allowing regeneration to begin.
They tracked this process over days. At first, the disc lost many key markers. Slowly, these markers returned. Genes like cut (ct), which controls wing margins, disappeared then reappeared. Other genes such as blistered (bs), which marks intervein regions, showed a similar pattern. The WNT family gene wingless (wg) also shifted its expression from broad damage signals back to precise developmental patterns.
By 72 hours after damage, most markers were restored. But when Zelda was inactivated, this restoration failed.
How Zelda Shapes Regrowth
Zelda ensures the tissue knows where veins, margins, and bristles belong. Without it, wings formed with blisters, missing veins, or reversed cell fates. The team found Zelda works alongside other pioneer factors, like Forkhead (Fkh) and GAGA factor (GAF). These factors often bind the same genome regions. If Zelda is lost, these partners cannot fully take over, leading to mistakes.
Using a method called CUT&RUN, researchers saw that Zelda binds genes that decide wing shape, vein placement, and cell identity. They also found that Zelda-bound regions become more accessible during regeneration, showing Zelda’s pioneer role in opening genes needed for healing.
Zld regulates margin, vein, and sensory organ fate after regeneration. (CREDIT: Science Advances)
In normal development, Zelda is not required for wing formation. This shows regeneration and development use different rules. Healing damaged tissues needs extra help to finish correctly.
This knowledge offers hope for medicine. Regenerative therapies aim to add factors that drive regrowth. But this carries risks. If new cells grow in the wrong way, it could cause cancer or birth defects. Understanding factors like Zelda teaches scientists how to promote healing safely.
Unlocking Future Therapies
Smith-Bolton’s team is now exploring if increasing Zelda during normal development could cause harm. They also want to know how Zelda is kept under control when regeneration is not happening. This work is supported by the NIH and the University of Illinois.
The story of Zelda teaches a simple truth. Healing is not just about starting the process. Ending it the right way is just as important. Nature balances growth with restraint to build perfect forms. By learning from tiny flies, we may one day guide human tissues to heal without error.