The discovery that a long, skinny fish can regrow its fin in a matter of weeks suggests that ancient vertebrates had considerable regenerative powers.
Two species of bichir from Africa can regrow amputated bony fins with remarkable accuracy, says developmental biologist Luis Covarrubias of the National Autonomous University of Mexico in Cuernavaca. Among the most ancient of the living lineages of ray-finned fishes, a group that includes most fresh- and saltwater species, the Polypterus bichirs share traits such as paired lungs with both modern amphibians and very early four-limbed vertebrates.
The venerable fishes’ powers suggest that early vertebrates shared substantial limb regeneration capability during the ancient evolutionary transition from fins to feet, Covarrubias and his colleagues contend in a paper published online February 21 in the Proceedings of the National Academy of Sciences. Those first steps toward life on land took place at least 375 million years ago.
Coauthor Rodrigo Cuervo, now at Veracruz University in Mexico, discovered the bichirs’ powers while following his curiosity about regeneration. The fish can go from zero to a full-size new side fin within a month. Many vertebrates, including mammals, can’t regenerate limbs at all. Biologists would love to understand why such a handy trait appears to have faded away in the course of evolution, or how it arose in the first place. “The real question is not why regeneration was lost but why it was ‘won,’ ” Covarrubias says.
Among animals without backbones, limb regeneration isn’t so startling. But only select groups of vertebrates living today can manage. Axolotls and other amphibians in the group of newts and salamanders can to varying degrees replace a lost limb, as can some other fish.
“Zebrafish are great at fin regeneration,” says Ken Poss of Duke University, who studies them. But their fins, as do fins of many other fish, contain mostly bones related to the fishes’ specialized, hardened skin. Bichir fins grow considerable fleshy tissue as well as bones of the type in the internal skeleton. Their comeback fins may prove useful for comparing regeneration systems, Poss says.