A tubular carnivore lives in rivers and streams around the world. It paralyzes and captures prey with a crown of tentacles, then draws it in through its mouth (which also serves as its anus). This disturbing creature is a hydra, a freshwater cnidarian no more than an inch long that eats mostly insect larvae and crustaceans. The mere appearance and eating habits of a hydra give it a sci-fi feel, but the ability to regenerate its body—even its head—from just a patch of tissue or a bunch of cells takes it to another level.
“It’s one of these organisms that’s thought never to die unless you try to kill it or, you know, starve it,” said Ali Mortazavi, a developmental biologist at the University of California, Irvine. A hydra’s regenerative ability allows it to constantly replace bits of itself, so it doesn’t succumb to things like old age or illness. Aside from the immortality benefit, constant regeneration means a hydra doesn’t have to sweat the little things like losing body parts. Give it a few days and it will grow back slightly.
dr. Mortazavi and his colleagues have taken a big step towards understanding how a hydra regenerates its head. Their research was published Wednesday in Genome Biology and Evolution.
To investigate what makes this remarkable feat possible, the researchers looked at changes in gene expression — whether a gene is copied from DNA to RNA — during hydrahead regeneration. This control of gene expression is called epigenetic regulation. Hydras have a genome quite similar to that of species with little regenerative capacity, such as humans, so epigenetic regulation is thought to play an important role in enabling the hydra’s regenerative capacity.
The team discovered dynamic changes in the regulation of stretches of DNA called enhancers. Enhancers increase the chances of a related gene being copied from DNA to RNA. These enhancers helped ensure the expression of many genes, the team found, including those long known to be important for regeneration. “No one knew that hydras had these strengthening regions,” said Dr. Mortazavi, who noted that the study placed hydra in the same club as many other animals, including mammals.
The researchers then compared gene expression during head regeneration with gene expression during budding, a form of asexual reproduction in which a hydra grows a polyp that is basically a copy of itself. That process requires the growth of a second head, but the researchers found that a budding head forms in a very different way than a head that regrows after injury.
“When I looked at the trends in gene expression, the genes increase a little bit slowly during nascent head development, but during regeneration we noticed these sharp twists and turns,” said Aide Macias-Muñoz, a developmental biologist at the University of California, Santa Barbara, who was one of the authors of the study. “A lot of genes are turned on and turned off and turned on again. So even though the end result is the same, it seems that the trajectory is actually very different.”
dr. Mortazavi was also surprised to find that the timing of gene expression varied so much between head regeneration and budding. “Obviously there’s more than one way to cope,” he said.
The discovery of these enhancer regions and their role in hydrakop regeneration also suggests that the evolution of enhancers predates the evolutionary divergence of cnidarians and bilaterians (animals with bilateral symmetry, such as humans) about 750 million years ago. Erin Davies, a developmental biologist at the National Cancer Institute who studies regeneration but was not involved in the work, sees these findings as a reminder of the importance of studying ancient creatures like hydras.
“They’re really in a great position to answer a lot of very fundamental questions in developmental biology,” she said, including “How did nervous systems evolve? How did you come up with bilateral symmetry?”
This kind of work is also essential for regenerative medicine, said Dr. Davies, where a common goal is to repair diseased and damaged tissues or even entire organs.
“Once you get a good grasp of a paradigm in an animal system,” she said, “you can start thinking about how you could reverse engineer things in less regeneration-competent species, like mammals.”