Science Spotlight: What Doesn't Kill These Fish Makes Them Whole Again

What Doesn't Kill These Fish Makes Them Whole Again 
By Akshaya Ramanujam

Some animals never got the memo that regeneration is supposed to be hard. The axolotl regrows an entire limbas though replacing a leg is a minor inconvenience. The zebrafish rebuilds its fins efficiently, and the Polypterus senegalus – an armored, lung-breathing relic of a fish whose body plan is virtually unchanged since the Devonian period 400 million years ago – can rebuild lost anatomy with the same unhurried competence. For decades, evolutionary biologists have wondered whether the ability to regenerate a complex appendage is something each lineage invented independently, or whether it is written into a much older, deeper genetic inheritance (Goss, 1969; Tanaka, 2016). A new study from the Schneider laboratory at Louisiana State University reveals that appendage regeneration in axolotls, zebrafish, and Polypterus is built on a shared, core genetic architecture inherited from a common vertebrate ancestor (Sousa et al., 2026).

The basic cellular choreography of regeneration has been mapped in detail. A wound triggers rapid clotting, followed by the assembly of a blastema, a mass of proliferating progenitor cells that collectively reconstructs the missing structure with absolute fidelity. Researchers also knew that parts of the genetic program used to build a limb during embryonic development are simply reactivated when that limb needs to be rebuilt during adulthood (Muneoka et al., 2008; Stocum, 2017). What remained strangely unresolved was whether the core of this process is genuinely shared across distantly related vertebrates, or whether each lineage simply arrived at a similar biological destination using entirely different molecular pathways (i.e. convergent evolution). Answering that question required a rigorous, multi-species comparison at single-cell resolution across multiple layers of genomic data; a milestone that no one had managed to achieve until now. 

It turns out that the basis of regeneration is ancient, and it pre-dates the evolutionary split between fish and land animals. However, the true surprise of the discovery is how flexible this blueprint is: each species executes the ancient program through its own distinct cellular strategies (Sousa, Lima et al., 2026). Some of these adaptations are predictable, others are deeply strange, and at least one involves a common muscle protein that has no obvious business being where researchers found it to be. 

The inclusion of Polypterus in this comparison was the linchpin of the project. Its paired fins contain endochondral bone in a structural arrangement far more reminiscent of a human arm or leg than the delicate dermal ray skeleton of a zebrafish fin (Zhu et al., 2012). Furthermore, Polypterus carries a clean, one-to-one genetic relationship with land animals. This contrasts sharply with zebrafish, which underwent a whole-genome duplication early in their evolution and now harbors multiple, subtly divergent copies of many genes (Braasch et al., 2016). In practical terms, tracking a signaling pathway in Polypterus meant identifying a single gene. In zebrafish, it meant arbitrating between several transcripts. According to co-first author Josane Sousa: "It was much easier to go through the annotation process with Polypterus."  

Unlocking these insights required the team to build every experimental protocol entirely from scratch. Polypterus senegalis, a foot long and covered with thick enamel-like ganoid scales, is no standard model organism. No commercial kit exists for it. Co-first author and PhD student Gabriela Lima describes the painstaking process with seasoned pragmatism: "I like to think of this project as many small victories over the years, from getting those animals in the lab to establishing those techniques on them." Combining single-nucleus RNA sequencing, bulk transcriptomics, and chromatin profiling across zebrafish, axoloyl and Polypterus at strict post-amputation intervals, the team painstakingly aligned the data. The results were striking: all three species deploy the exact embryonic program that drives initial limb outgrowth, proving the ancient origin of this regenerative strategy (Zhong et al., 2023; Nacu et al., 2016). 

Then came the unexpected breakthrough: The researchers noticed an unusual population of red blood cells expanding dramatically after bone amputation in both Polypterus and axolotls, though these cells were entirely absent in the zebrafish. These cells expressed a previously hidden oxygen-sensing gene, suggesting that oxygen levels actively govern regeneration. More puzzling still, myoglobin, a protein known for storing oxygen inside muscle tissue, was unexpectedly upregulated in the basal epidermis of the wound. This surprising finding sent two students in the lab down an entirely new line of investigation. 

The picture that ultimately emerges is of regeneration as a highly modular system. There is an ancient, shared operating system that boots up whenever an appendage is lost. Yet, the cellular implementation of that system is incredibly plastic. Says Lima: "Appendage regeneration is built from a deeply conserved genetic program, but different species deploy this program in flexible ways." 

This flip in perspective carries profound implications for non-regenerating vertebrates, like mammals. The right question to ask about humans or mice may not be what magical regeneration pathways we are missing, but rather which parts of this deeply embedded ancestral program have been silenced, repurposed, or quietly dismantled over evolutionary history (Seifert and Muneoka, 2018; Lozito and Tuan, 2015). The Schneider lab has already spent four years building a parallel dataset in non-regenerating species to pin down exactly where the process stalls. By chasing the oxygen-sensing thread, linking the erythrocyte population, the hypoxia-inducible factor, and an unexpected epidermal myoglobin, they hope to discover whether oxygen-sensing is the ultimate gatekeeper for engineering regenerative success in species that lost the ability millions of years ago. 

   

summary scheme, Sousa, Lima et al., 2026

 

About the Authors

Josane Freitas Sousa is a senior postdoctoral researcher in the Schneider laboratory who transitioned to regeneration biology after a PhD studying cancer, a field she aptly describes as an injury response gone wrong. Thorough to a fault, she is the kind of scientist who dissects a protocol the night before, to anticipate what may go wrong at step seven. Outside the lab, Sousa channels her love for strategy into complex board games with her daughter, masters the chemistry of cooking, and is currently preparing for her next major milestone: stepping into a research assistant professor position. 

Gabriela Lima is a PhD student in the Schneider laboratory and co-first author of the study, driven by a fascination with how evolution meticulously shapes the genetic regulatory networks behind regeneration. Bringing that same strategic focus to her life away from the microscope, she is the person most likely to convince you that running, football, and volleyball are not actually three separate hobbies, but rather a single, coherent lifestyle. 

Igor Schneider, the corresponding author, is an Assistant Professor in the Department of Biological Sciences at Louisiana State University. He leads his lab with an infectious enthusiasm for evolutionary questions and a level of hands-on involvement that keeps his team inspired. Known for spotting deep evolutionary patterns faster than most people can finish their morning coffee, he commands a rare and genuine warmth from his team. Both co-first authors describe a collaborative environment where high-speed insights meet heartfelt mentorship. 

 

About the Science Writer

Akshaya Ramanujam is a doctoral researcher at the Medical University of Lublin, Poland, where she uses zebrafish to study hereditary human limb malformations. Drawn to the stubborn puzzles where DNA meets real life, she has successfully secured a career with zero shortage of challenges. Outside the lab, Akshaya doodles abstract art and debates Formula 1 strategy, firmly believing that aerodynamic efficiency and genetic mutations are simply different ways of navigating structural design constraints. 

 

References

1. Muneoka K, Han M, Gardiner DM. Regrowing human limbs. Sci Am. 2008 Apr;298(4):56-63. doi: 10.1038/scientificamerican0408-56. PMID: 18380142. 

2. Tanaka EM (2016). The molecular and cellular choreography of appendage regeneration. Cell, 165, 1598-1608. 

3. Goss RJ (1969). Principles of Regeneration. Academic Press, New York. 

4. Braasch I et al. (2016). The spotted gar genome illuminates vertebrate evolution and facilitates human-teleost comparisons. Nature Genetics, 48, 427-437. 

5. Zhong J, Aires R, Tsissios G et al. (2023). Multi-species atlas resolves an axolotl limb development and regeneration paradox. Nature Communications, 14, 6346. https://doi.org/10.1038/s41467-023-41944-w 

6. Nacu E et al. (2016). FGF8 and SHH substitute for anterior-posterior tissue interactions to induce limb regeneration. Nature, 533, 407-410 

7. Seifert AW, Muneoka K (2018). The blastema and epimorphic regeneration in mammals. Developmental Biology, 433, 190-199. 

8. Sousa JF, Lima G et al. (2026). Comparative multi-omic analysis reveals conserved and derived mechanisms of fin and limb regeneration. Nature Communications. DOI: https://doi.org/10.1038/s41467-026-68801-w 
   Stocum DL (2017). Mechanisms of urodele limb regeneration. Regeneration, 4, 159-200. 

9. Tanaka EM (2016). The molecular and cellular choreography of appendage regeneration. Cell, 165, 1598-1608. 

10. Zhu M et al. (2012). Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian. Nature Communications, 3, 772.