Hydra DNA reveals there’s more than one way to regrow a head

In rivers and streams across the globe lives a tube-shaped carnivore. It paralyses and captures prey with a crown of tentacles, then draws it in through its mouth (which also serves as its anus). This unsettling creature is a hydra, a freshwater-dwelling cnidarian no more than a half-inch long that eats mostly insect larvae and crustaceans.

“It’s one of these organisms that’s thought to never die unless you try to kill it or, you know, starve it to death,” Ali Mortazavi, a developmental biologist at the University of California, Irvine, said. A hydra’s regenerative abilities allow it to constantly replace bits of itself, so it doesn’t succumb to things like old age or disease.

Mortazavi and his colleagues have taken a big step in understanding how a hydra regenerates its head. Their research was published 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 into RNA — throughout the course of hydra head regeneration. This control of gene expression is called epigenetic regulation. Hydras have a genome quite similar to that of species with little regenerative capacity, like humans, so it’s thought that epigenetic regulation plays a major role in making the hydra’s powers of regeneration possible.

The team discovered dynamic alterations in the regulation of stretches of DNA called enhancers. Enhancers increase the likelihood that a related gene will be copied from DNA into RNA. These enhancers were helping to ensure the expression of many genes, the team found, including those long known to be important for regeneration. “Nobody knew hydras had these enhancer regions,” said Mortazavi, who noted that the study put 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 is basically a copy of itself. That process requires growth of a second head, but the researchers found that a budding head forms in a very different way from a head regrowing after injury.

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 (animals with bilateral symmetry, like humans) around 750 million years ago

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