Biologists from the University of Bergen and the University of Vienna have studied a simple, brainless sea anemone to learn more about the evolutionary origin of the head.
In many animals, the brain is located in a specific structure, the head, together with sensory organs and often together with the mouth. However, there are even more distantly related animals, which have a nervous system, but no brain, like sea anemones and corals.
The scientists used the Starlet sea anemone Nematostella vectensis to find out if one of the ends of the creature corresponds to the head of higher animals. To do this, they studied the function of genes that control head development in higher animals during the embryonic development of Nematostella. Their findings, published in the open-access journal PLoS Biology, show that the same genes that control head development in higher animals regulate the development of the front end of swimming Nematostella larvae.
“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,” explained study co-author Dr Ulrich Technau the University of Vienna’s Center of Organismal Systems Biology.
When the sea anemones are in the larval stage they swim and search for a suitable site for settlement and metamorphosis into a polyp. During this metamorphosis, it is the front end of the larva, which senses the environment and attaches to the ground, while the other end, transforms into the oral side of the animal with mouth and tentacles.
The researchers found that the differentiation of the aboral front of the sea anemone larva is governed by a hierarchy of genes, controlled by an upstream master control gene called Six3/6. Notably, this gene as well its downstream dependent genes, also play a crucial role in setting up the field for making the brain of flies, fish and human. 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. And indeed, the ‘head genes’ function on this side of the animals, yet they are not used to form a full brain,” said senior author Dr Fabian Rentzsch of the University of Bergen.
“Sea anemones and all higher animals, including humans, share a common brainless ancestor, which lived between 600 and 700 million years ago.”
By revealing the function of ‘head genes’ in Nematostella, the scientists believe that they now understand better how and from where the head and brain of higher animals evolved. Definitely, the gene network that formed a sensory center has been evolved in this common ancestor some 600 million years ago.
“Based on the appearance of the adult animals, the aboral 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,” said Dr Technau.
Bibliographic information: Sinigaglia C et al. 2013. The Bilaterian Head Patterning Gene six3/6 Controls Aboral Domain Development in a Cnidarian. PLoS Biol 11(2): e1001488; doi: 10.1371/journal.pbio.1001488