"Long before it's in the papers"
February 22, 2013

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Humans may be “upside-down” with respect to jelly ancestors

Feb. 22, 2013
Courtesy of the University of Vienna
and World Science staff

The ev­o­lu­tion­ary or­i­gin of the head is trace­a­ble to a brain­less or­gan usu­ally con­sid­ered the “foot” of crea­tures re­lat­ed to jel­ly­fish, sci­en­tists re­port.

Re­search­ers found that the same genes that con­trol head de­vel­op­ment in high­er an­i­mals gov­ern the de­vel­op­ment of the front end of the swim­ming lar­va, or ju­ve­nile form, of a sim­ple, brain­less sea anem­o­ne. This front end be­comes the so-called “foot” of the adult an­i­mal.

An adult starlet sea ane­mone (Ne­ma­to­stel­la vec­ten­sis) swim­ming freely.
In many an­i­mals, the brain is lo­cat­ed in the head, to­geth­er with sen­so­ry or­gans and of­ten the mouth. But there are dis­tantly re­lat­ed an­i­mals that have a nerv­ous sys­tem, but no brain, like sea anem­o­nes and corals.

The re­search group, led by Fa­bi­an Rentzsch of Sars Cen­tre in Ber­gen, Nor­way and Ul­rich Tech­nau from the Uni­vers­ity of Vi­en­na, set out to find if one of the ends of the sea anem­o­ne cor­re­sponds to the head of high­er an­i­mals. The sci­en­tists stud­ied the func­tion of genes that con­trol head de­vel­op­ment in high­er an­i­mals dur­ing the em­bry­on­ic de­vel­op­ment of the star­let sea anem­o­ne.

“De­spite look­ing com­pletely dif­fer­ent, it has be­come clear over the last dec­ade that all an­i­mals have a si­m­i­lar rep­er­toire of genes, in­clud­ing those that are re­quired to make the head of high­er an­i­mals,” said Tech­nau.

The study is pub­lished in the jour­nal PLoS Bi­ol­o­gy.

When the sea anem­o­nes are in the lar­val, or ju­ve­nile, stage they swim and search for a suit­a­ble site to settle and met­amor­phosize in­to a form called a pol­yp. Dur­ing this pro­cess, the front end of the lar­va senses the en­vi­ron­ment and at­taches to the ground. The oth­er end be­comes in­to the “oral” side of the an­i­mal with a mouth and ten­ta­cles for bring­ing in food.

The re­search­ers found that the lar­va’s front-end de­vel­op­ment is is gov­erned by a hi­er­ar­chy of genes, con­trolled by the “mas­ter con­trol gene” called Six3/6. No­ta­bly, this gene and genes that de­pend on it “down­stream” al­so play a cru­cial role in set­ting up the field for mak­ing the brain of flies, fish and men, Tech­nau and col­leagues ex­plained. Hence, the func­tion of the “head genes” is lo­cat­ed at the end that cor­re­sponds to the “foot” of the adult ane­mone. 

“The an­te­ri­or end of the swim­ming lar­va car­ries their main sense or­gan, so at this stage it looks more like this might be their head,” said Rentzsch. In­deed, the “head genes” func­tion on this side of the an­i­mals, he added, yet they are not used to form a full brain.

Sea anem­o­nes and all high­er an­i­mals, in­clud­ing hu­mans, are be­lieved to share a com­mon brain­less an­ces­tor that lived be­tween 600 mil­lion and 700 mil­lion years ago. By re­veal­ing the func­tion of “head genes” in the anem­o­ne, the sci­en­tists be­lieve they now un­der­stand bet­ter how and from where the head and brain of high­er an­i­mals evolved. Def­i­nite­ly, they said, the gene net­work that formed a sen­so­ry cen­ter had evolved in this com­mon an­ces­tor some 600 mil­lion years ago.

“Based on the ap­pear­ance of the adult an­i­mals, the ab­o­ral [at­tach­ing] end of these an­i­mals has tra­di­tion­ally been called the foot and the up­per end the head, while in fact it is bas­ic­ally turned up­side down,” ex­plains Tech­nau. “Or we...” he added with a smile.

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The evolutionary origin of the head is traceable to a brainless organ usually considered the “foot” of creatures related to jellyfish, scientists report. Researchers found that the same genes that control head development in higher animals govern the development of the front end of the swimming larva, or juvenile form, of a simple, brainless sea anemone. This front end becomes the “foot” of the adult animal. In many animals, the brain is located in the head, together with sensory organs and often the mouth. But there are distantly related animals that have a nervous system, but no brain, like sea anemones and corals. The research group, led by Fabian Rentzsch of Sars Centre in Bergen, Norway and Ulrich Technau from the University of Vienna, set out to find if one of the ends of the sea anemone corresponds to the head of higher animals. The scientists studied the function of genes that control head development in higher animals during the embryonic development of the starlet sea anemone. “Despite looking completely different, it has become clear over the last decade, that all animals have a similar repertoire of genes, including those that are required to make the head of higher animals,” said Technau. The study is published in the journal PLoS Biology. When the sea anemones are in the larval, or juvenile, stage they swim and search for a suitable site for settlement and metamorphosis into a form called a polyp. During this transformation, the front end of the larva senses the environment and attaches to the ground. The other end becomes into the “oral” side of the animal with a mouth and tentacles for bringing in food. The researchers found that the larva’s front-end development is is governed by a hierarchy of genes, controlled by the “master control gene” called Six3/6. Notably, this gene as well as genes that depend on it “downstream,” also play a crucial role in setting up the field for making the brain of flies, fish and men, Technau and colleagues explain. Hence, the function of the “head genes” is located at the end that corresponds to the “foot” of the adult animals. “The anterior end of the swimming larva carries their main sense organ, so at this stage it looks more like this might be their head,” said Rentzsch. Indeed, the “head genes” function on this side of the animals, he added, yet they are not used to form a full brain. Sea anemones and all higher animals, including humans, are believed to share a common brainless ancestor that lived between 600 million and 700 million years ago. By revealing the function of “head genes” in the anemone, the scientists believe they now understand better how and from where the head and brain of higher animals evolved. Definitely, they said, the gene network that formed a sensory center had evolved in this common ancestor some 600 million years ago. “Based on the appearance of the adult animals, the aboral [attaching] end of these animals has traditionally been called the foot and the upper end the head, while in fact it is basically turned upside down,” explains Technau. “Or we...” he added with a smile.