Regrowing a severed head is no problem for sea creatures fittingly called Hydra, and researchers have now mapped exactly how this happens at a genetic level for the first time.
Hydra are a group of small, tree-shaped fish. Unlike mammals, they are loaded up with stem cells that have the unlimited capability to renew themselves, and so are commonly believed to be “biologically immortal”.
They reproduce asexually in a process called budding, where the offspring grows headfirst from the base of the animal.
Because this is such an uncommon trait, the networks of genes used to regrow entire organs – including the head – remain largely unexplored.
It’s all epigenetic
Now, a team of researchers has found that Hydra regrow severed heads in a very different way to how they bud. It wasn’t the types of genes that matter, but how the genes are used – a phenomenon known as epigenetics.
Genetic processes are complex. Stem cells can turn into any other cell, depending on which genes are expressed during development.
Genomics DNA holds all the information about every gene, but not every gene is expressed in a single cell. Some of the DNA is turned into messenger RNA (mRNA) through a process called transcription, which is then translated into a chain of amino acids. These chains fold into proteins, which are responsible for carrying out work in the cell.
The final cocktail of proteins determines what type of cell the stem cell can turn into.
Both transcription and translation require special regulatory elements to be successful. These elements can also dictate which genes are switched on and off to control the quantity of protein made, which can have a huge effect on the fate of stem cells. This is the driving principle behind epigenetics.
How Hydra regrow their heads
In their study, published in Genome Biology and Evolution, the researchers identified around 13,000 regulatory elements that were remodelled during Hydra head regeneration. These elements switched a small subset of genes on and off differently during head regeneration than at other times, altering the population of proteins that were eventually made.
To illustrate how this works, imagine the cell is like a large production factory. The transcription factors are like managers who instruct the workers on their tasks. They can also tell the workers to divert their work and focus on a different process for a little while. The tools they use never change, but they are now working with different instructions about what to make, and they will product a different product – in this case, it’s a brand-new head.
“One exciting finding of this work is that the head regeneration and budding processes in Hydra are quite different,” says the paper’s lead author, Aide Macias-Muñoz. “Even though the result is the same (a Hydra head), gene expression is much more variable during regeneration.
“These findings suggest that complex developmental enhancers were present early in evolution.”
Deborah Devis is a science journalist at Cosmos. She has a Bachelor of Liberal Arts and Science (Honours) in biology and philosophy from the University of Sydney, and a PhD in plant molecular genetics from the University of Adelaide.
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