Hydra DNA reveals there’s more than one way to grow a bud

In rivers and streams around the world lives a tube-shaped carnivore. It paralyzes and captures its prey with a crown of tentacles, then attracts them through its mouth (which also serves as its anus). This disturbing creature is a hydra, a freshwater cnidarian up to half an inch long that feeds primarily on insect larvae and crustaceans. A hydra’s appearance and eating habits alone give it a sci-fi feel, but its ability to regenerate its body – even its head – from a single piece of cloth or a pile of cells take it to another level.

“It’s one of those organisms that you think will never die unless you try to kill it or, you know, starve it,” said Ali Mortazavi, development biologist. at the University of California at Irvine. A hydra’s regenerative abilities allow it to constantly replace pieces of itself, so that it doesn’t succumb to things like old age or disease. Aside from the benefit of immortality, constant regeneration means that a hydra doesn’t have to sweat the little things, like losing body parts. Give it a few days and it will push back anything.

Dr Mortazavi and his colleagues have taken a big step to understand 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 – if a gene is copied from DNA to RNA – throughout hydra head regeneration. This control of gene expression is called epigenetic regulation. Hydras have a genome quite similar to species with low regenerative capacity, such as humans, so epigenetic regulation is believed to play a major role in making the hydra’s regenerative powers possible.

The team discovered dynamic alterations in the regulation of DNA segments called enhancers. Enhancers increase the likelihood that a related gene will be copied from DNA into RNA. The team found that these enhancers help ensure the expression of many genes, including those long known to be important for regeneration. “No one knew that hydras had these amplifying regions,” said Dr Mortazavi, who noted that the study put the 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 where a hydra grows a polyp that’s essentially a copy of itself. This process requires the growth of a second head, but researchers have found that a budding head forms in a very different way than a head that grows back after injury.

“When I looked at the trends in gene expression, the genes sort of increase slowly throughout the development of the budding bud, but upon regeneration we noticed these sharp turns,” Aide said. Macias-Muñoz, a developmental biologist at the University of California, Santa Barbara, who was one of the study’s authors. “A lot of genes are turned on, then off, then back on. So even though the end result is the same, it looks like the trajectory is actually quite different. “

Dr Mortazavi was also surprised to find that the timing of gene expression varied so much between head regeneration and budding. “Clearly there is more than one way to freak out,” he said.

The discovery of these enhancer regions and their role in hydra head regeneration also suggests that the evolution of enhancers predates the evolutionary divergence of cnidarians and bilaterians (bilaterally symmetrical animals, such as humans) ago. about 750 million years old. Erin Davies, a developmental biologist at the National Cancer Institute who studies regeneration but has not been involved in the work, sees the findings as a reminder of the importance of studying ancient creatures like hydras.

“They are really in a privileged position to answer a lot of very fundamental questions in developmental biology,” she said, including “How has the nervous system evolved? How did you get the bilateral symmetry?

This type of work is also essential for the field of regenerative medicine, said Dr Davies, where a common goal is to restore diseased and injured tissue or even entire organs.

“If you have a good grasp of a paradigm in any animal system,” she said, “then you can start to think about how you might reverse the engineering of things in species that are less competent at regeneration, like mammals. “

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