Starting Anew: Scientists Turn to Lowly Creatures to Unlock the Secrets of Regeneration
Gaidos, Susan, Science News
It could have been a scene from a sequel to Jurassic Park: Peering down at the tiny worms wriggling under the lens of her microscope, biologist Alexandra Bely witnessed a performance that hadn't been played in nature in millions of years. The beastie was sprouting a second head.
Actually, two-headed worms are common in Bely's lab at the University of Maryland in College Park. But this specimen belongs to a species that had long ago lost the unusual regenerative ability.
That species, Paranais litoralis, is part of an ancient family of worms called naidids that settle in the soft sediments alongside streams and ponds. Generally, if a sudden rush of water or a hungry predator causes a naidid to lose its head, it will simply grow another one. But some species that Bely and colleagues have studied, including Pa. litoralis, seem to have lost this power. So it surprised Bely to see that, with the right timing, the creature could regain its head-popping potential.
"That's very exciting, because it indicates that the ability to regenerate is still there, in a dormant state," Bely says, "though it probably hasn't been expressed or seen in millions of years."
Bely's finding and other recent results have encouraged researchers who are trying to figure out why some animals can reconstruct their body parts while others can't. Most species have the ability to regenerate some body parts, yet this talent is highly variable. Humans, for instance, can renew skin and bone, but salamanders can re-create entire limbs or tails, or just about any other structure that can be lopped off without killing them. And the real superstars are animals such as sea stars, flatworms and sponges: They can regenerate every part of their body, even from a tiny fragment.
Given the obvious medical appeal of regrowing lost or injured body tissues, scientists have long focused on the molecular signals that drive regeneration--endowing some animals with extensive powers while countless others have limited or no regenerative abilities at all. In recent years, scientists have begun to explore the regenerative process in diverse creatures to better understand how that power evolved in the first place and why not everything has it.
The findings have raised a nagging question: Are the regenerative powers that lead to a new head in a worm, restore the leg of a salamander and heal the skin of a human one and the same? Some scientists maintain that regeneration is ancient and originated long ago, while others say it has evolved, again and again, in different organisms.
Bely says figuring out whether regenerative abilities in all animals had the same evolutionary origins is the "elephant in the room" for regeneration biology. "Whether or not regenerative processes are homologous across the animal kingdom profoundly affects how we interpret what we learn about regeneration in different animal groups."
In the past, researchers have sought a single explanation for the wide variation in regenerative abilities. Since animals across the board have some renewing potential, some scientists have suggested that complex creatures tended to lose the regenerative ability as evolution proceeded, while simpler animals retained it. Recent studies on the genes and signaling pathways involved in regeneration show that its presence or absence is driven by multiple factors, as Bely and colleague Kevin Nyberg describe in a paper to appear in Trends in Ecology & Evolution.
To determine whether all regeneration has the same roots, scientists must identify its molecular basis in different groups of organisms. Bely and other researchers discussed the evolutionary aspects of regeneration--as well as ecological and physiological factors that play into it--in January in Seattle at the Society for Integrative and Comparative Biology meeting.
"If you look at the Metazoan tree of life and look at all the phyla that have been described so far, you will find organisms that can do some remarkable regenerative feats," says Alejandro Sanchez Alvarado, a neurobiologist at the University of Utah in Salt Lake City who spoke at a panel session at the meeting. …