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Scientists recreate “Hydra” of myth, in miniature

Aug. 1, 2007
Courtesy PLoS
and World Science staff

In an ancient Greek myth, the he­ro Her­cu­les kills a many-headed marsh mon­ster called Hy­dra that could re­grow its heads if they were cut off.

Such beasts, of, course, don’t ex­ist. But sci­en­tists have found that they can cre­ate many-headed jel­ly­fish by de­ac­ti­vating cer­tain genes. Oddly enough, the ti­ny jel­ly­fish they stud­ied are close rel­a­tives of an an­i­mal gen­u­inely called Hy­dra; and both crea­tures be­long to a sci­en­tif­ic class known as Hy­dro­zoa. 

Eleuthe­ria with two heads (above; ar­rows point to heads) and sev­er­al heads (be­low). (Im­age cour­te­sy P­LoS One)


These names in­deed come from that of the myth­i­cal beast, as the real Hy­dra re­sem­bles the myth­i­cal one some­what. But not be­cause it has many heads. What it does have is many ten­ta­cles, and a great abil­ity to re­grow body parts or mul­ti­ply when cut.

Mul­ti­ple-headed an­i­mals were pre­vi­ously rec­og­nised as a rare an­o­maly of un­known or­i­gin, the re­search­ers said. 

But in the stu­dy, pub­lished in the Au­gust 1 is­sue of the on­line re­search jour­nal P­LoS One, they de­vel­oped a way to block genes known as Cnox in cnidar­i­ans, the group of an­i­mals that in­cludes Hy­dro­zoa, oth­er jel­ly­fish and var­i­ous oth­er sting­ing sea crea­tures. 

The Cnox genes—rel­a­tives of genes called Hox in high­er an­i­mals—are re­spon­si­ble for form­ing the body along its main head-to-tail ax­is.

The study led to a re­li­a­ble way to cre­ate man­y-headed jel­ly­fish, said the re­search­ers, Wolf­gang Ja­kob and Bernd Schier­wa­ter of the Un­ivers­ity of Vet­er­i­nary Med­i­cine Han­no­ver in Ger­ma­ny. The work pro­vid­ed in­sights in­to how genes gov­ern head forma­t­ion in some of the most prim­i­tive an­i­mals, they added.

Her­cu­les slays the Hy­dra in a 1545 en­grav­ing by Hans Se­bald Be­ham. In the Greek tale, an ac­com­plice burned the stumps of the mon­ster's heads be­fore they could grow back, lead­ing to the beast's fi­nal down­fall.


If a cer­tain Cnox gene is shut down, they found they could gen­er­ate hy­dro­zo­an jel­ly­fish of the spe­cies Eleu­the­ria di­cho­to­ma with two heads, that both fully func­tion and take in food. De­ac­tiva­t­ion of a dif­fer­ent gene led to even more heads, they added.

The rar­ity of such beasts in na­ture may be be­cause mul­ti­ple heads of­fer lit­tle or no ad­van­tage, Ja­kob and Schier­wa­ter said. 

Then again, they might not be so rare, de­pend­ing on how you look at it. Some cni­dar­i­ans form col­o­nies, such as those that build cor­al reefs, they not­ed. These can be seen as mul­ti­ple heads shar­ing a com­mon gut, or di­ges­tive sys­tem; they might have evolved from single-headed an­ces­tors.

Ja­kob and Schierwa­ter ac­knowl­edged, though, that they failed to rep­li­cate one key as­pect of the mon­strous hy­dra of lo­re. When­ev­er Her­cu­les de­cap­i­tat­es the Hy­dra, two new heads im­me­di­ately grew back. In the case of the man­y-headed Eleuthe­ria, only one head re­grows per lost head.


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In a tale from Greek mythology, the muscleman hero Hercules defeats a many-headed water monster called Hydra, which could regrow its heads if they were cut off. Such monsters, of, course, don’t exist. But scientists from the University of Veterinary Medicine Hannover, in Germany, have found that they can create many-headed jellyfish by deactivating certain genes. Oddly enough, the tiny jellyfish they studied are close relatives of an animal genuinely called hydra; and both creatures belong to a scientific class known as hydrozoa. These names indeed come from that of the mythical beast, as the real hydra resembles the mythical one somewhat—but not because it has many heads. What it does have is many tentacles, and a great ability to regrow body parts or multiply when cut. Multiple-headed animals were previously recognised as a rare anomaly of unknown origin, the researchers said. But in the study, published in the August 1 issue of the online research journal PLoS ONE, they developed a way to block genes known as Cnox in cnidarians, the group of animals that includes hydrozoans, other jellyfish and various other stinging sea creatures. The Cnox genes—relatives of genes called Hox in higher animals—are responsible for forming the body along its main head-to-tail axis. The study led to a reliable way to create many-headed jellyfish, said the researchers, Wolfgang Jakob and Bernd Schierwater. The work provided insights into how genes govern head formation in some of the most primitive animals, they added. If a certain Cnox gene is deactivated, they found they could generate hydrozoan jellyfish of the species Eleutheria dichotoma species with precisely two heads, that both fully function and take in food. Deactivation of a second gene leads to multiple heads. The rarity of such beasts in nature may be because multiple heads offer little or no advantage, Jakob and Schierwater said. Then again, they might not be so rare, depending on how you look at it. Some cnidarians form colonies, such as those those that build coral reefs, they noted. These can be seen as multiple heads sharing a common gut, or digestive system; they might have evolved from single-headed ancestors. Jakob and Schierwater acknowledged, though, that they failed to replicate one key aspect of the monstrous hydra of lore. Whenever Hercules decapitated the Hydra, two new heads immediately grew back. In the case of the many-headed Eleutheria, only one head regrows after decapitation.