Issue 7

When we think of a Darwinian Demon, it is easy to imagine an oversized, terrifying monster, like something out of an alien movie, ready to take over the planet.  A species that is able to reproduce from birth, indefinitely, for the rest of its immortal life, running rampant over the face of the earth (with its many, many relatives) and destroying everything in sight. What does not spring to mind is a small jellyfish blissfully bobbing along in the deep blue sea. Yet Turritopsis dohrii (formally Turritopsis nutricula), also known as the immortal jellyfish, is exactly that. While it might not be a fully-fledged Darwinian Demon, this 5 millimetre long sea dweller has a life history to be reckoned with. It is capable of ontogeny reversal, that is, developing backwards until it is once again a sexually immature polyp on the sea floor. It has the potential to live forever, with its reproduction limited only by available the resources.

 

The typical lifecycle of a jellyfish is pretty simple. When a free-floating egg is fertilised, it develops into a larva and sink to the sea floor, where it attaches itself to something. Thus the jellyfish grows and begins its life as an asexual, colonel organism known a polyp. From here, the polyp ‘buds’ in to an adult medusa, the solitary bell-shape organisms that we associate with jellyfish. These medusa are sexual and can realise their own free-floating gamete ready to form the next generation.   

 

In most animal species, mother and father would now grow old and die, safe in the knowledge they have at least managed to pass their genetic legacy on to their offspring. The immortal jellyfish, however, settles in for round two (… or three… or four…). Reversing development is also usefully during times of environmental stress, such when food is scarce or when the jelly fish is injured.  Medusa, sexually mature or not, can revert back into polyps and wait for things to improve.

 

But just how do they do it? Ontogeny reversal is the product of two processes: cell transformation and cell transdifferentiation. Transdifferentiation is the process by which a specialised cell is able to wipe clean any ‘instructions’ it received during differentiation and transform into another type of cell with an entirely different function. Transdifferentiated cells are able to alter their genetic make-up, similar to stem cells, allowing them to revert back to a former state. For a medusa to convert back into a polyp, the remaining jellyfish fragment must contain tissue of the exumbella (the top ‘umbrella’ of the jellyfish) and the canal system, but it remains unclear exactly which cells are required to replace any missing cell types (such as sensory or myoepithelial cells).

 

First, the bell of the medusa is inverted and the tentacles and mesoglea are reabsorbed. This leaves a ball of tissue, called a cyst, which once again sinks to the seafloor. At temperatures of around 22 °C branched stem-like structures, called stolons, form 2 to 3 days later. At lower temperatures, cysts can rest for up to three months before temperature rises again and stolon production can commence. Another 2 days of ‘un-development’ and the polyp arises, now able to feed on brine shrimp and hopeful once again bud new medusa.

 

 

 

 

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While it is reported several other species can achieve this amazing feat, such as Podocoryne carnea, Eleutheria dichotoma, Cladonema ichidai and Perarella schneideri, T.dohrii is the only metazoan capable of making the return journey after having already reached a state of sexual maturity. This means it can revert back to infancy and grow up once again alongside its offspring, and potentially its offspring’s offspring. So why is it we are not drowning in a sea of these jellyfish?

 

Theoretically, the process of transdifferentiation can occur indefinitely, with laboratory studies showing individuals reverting back to polyps 100% of the time. However, although it is unlikely such transformation potential is an artefact of laboratory conditions, there is currently no evidence of this process occurring in nature. Also, despite having no natural somatic death, T.dohrii are still subject to predation and disease.

 

The amazing life-history of Turritopsis dohrii could have huge potential for medical studies, providing possibilities such as organ reproduction, treatment of brain injuries, and even a potential cure for cancer. By switching off genes in cancerous cells, such as in transdifferentiation, it might be possible to reset cells to early, healthy, stages of their life cycle! Now admit it, T.dohrii is hardly the foul, monstrous Darwinian Demon you were expecting, is it?