A Unique Genetic Regulation Helps Hydras Regrow Its Own Head

Despite the simplicity of their appearance, the aquatic creatures (Hydra) possess supernatural and amazing powers – including the absurd ability to regrow their own heads if decapitated.

Simple and amazing creatures

Hydra belongs to the cnidaria phylum of animals, which includes jellyfish, corals and sea anemones, the 10-20 mm hydra live in freshwater tropic and temperate environments.

These invertebrate fresh water animals are also one of the closest examples we have of an immortal being. Unless thoroughly destroyed within a predator’s digestive system or consumed by fire, hydra stem cells can replicate indefinitely.

Hydra can regenerate and grow itself if it is exposed to any danger as long as there are at least 5 of the cells that regulate the growth of the head intact and have not undergone any rupture, as these few cells can rearrange the scene and repair any chaos that occurred in the hydra’s body until it grows again into a complete organism.

Pureed hydra cells regaining form over time with new head organizer cells (blue).

These incredible traits have long held scientists’ attention, but there’s still so much about how they work that remains mysterious.

The bodies of Hydra are radially symmetrical, like other organisms belonging to the phylum Cnidaria, which is different from the bilateral symmetry of our bodies, which means that the external body parts of the animal correspond in an orderly manner.

Organized cells

At the hydra’s head end, between its ring of tentacles is its dome-shaped hypostome – a structure that becomes their mouth when they rip themselves open to eat. The cells seal back together when not in use, and within this same structure is where those 50–300 head organizer cells usually reside.

These cells direct their neighboring cells to form the head of the hydra, and that is by defining the cellular signals that push the neighboring cells either to form the hypoostome, or to form the arms.

If the hydra is chopped in two, anywhere along its top third, the remaining body bit will grow more organizer cells, which will then arrange a shiny new head for the animal.

These command cells also appear naturally along the hydra body when it is budding, reproducing asexually.

These amazing features have long attracted scientists’ attention, as we still don’t know much about how these cells function. So, University of California biologist Ed Macias-Muñoz and her team examined the genes of the hydra and studied the genes that are expressed during the process of head regeneration and budding.

free-living Hydra viridissima organism

Genetic differentiation

Previous research suggested the epigenetics of multiple developmental pathways are involved, referring to how genes in these pathways are regulated. Messing with some of the regulatory genes can produce some bizarre results, like multiple head organizers along the hydra’s body.

According to the report published by Science Alert, in response to the study, which was recently published in the journal Genome Biology and Evolution on December 8, Macias-Muñoz says that “One exciting finding of this work is that the head regeneration and budding programs in Hydra are quite different.”

“Even though the result is the same (a hydra head), gene expression is much more variable during regeneration. Accompanying dynamic gene expression is dynamic chromatin remodeling at sites where developmental transcription factors bind.”

In other words, the scaffolding that DNA coils around for structure – its chromatin – is opened up at these regions to allow the cells to make use of these developmental genes.

Scientists have identified many regions of the genome – totaling 2,870 regions – that were active within the cells that regulate head regeneration, as the products of these genes help in other growth processes.

These findings suggest these complex developmental enhancers were present before Cnidaria evolutionarily split from the group of animals that are bilaterally symmetrical (which includes us), 600 million years ago, explains Macias-Muñoz.

The team also found that a distinct family of genes known as Fos was also involved in the process of head regeneration. Notably, this family sees regeneration processes in a number of other species including fish, salamanders and mice.

The researchers also note that the genes of cnidarians such as the hydra are surprisingly similar to our own, which means that the morphological differences that distinguish them from us are likely a product of how these genes are regulated.