The Brainless Origin of The Head

The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian. 2013. Sinigaglia, C., et al. PLoS Biology 10.1371/journal.pbio.1001488.

In a simple, brainless sea anemone, the same genes that control head development in higher animals regulate the development of the front end of the swimming larvae. link

Author Summary [edit]: The evolutionary origin of head development is a fundamental question for understanding the evolution of animal body plans. Bilaterally symmetrical animals (Bilaterians) have an anterior-posterior (head-to-tail) axis, whose anterior end is usually characterized by a head and a brain.

Bilaterians evolved from an ancestor shared with cnidarians (corals, sea anemones, jellyfish), but brain-like structures are absent in cnidarians, although they have an obvious oral-aboral axis. Cnidarian larvae move with the aboral pole forward, but as adult polyps this pole is anchored to the ground, while the oral end is used for feeding. It is unclear whether one of the termini of cnidarians corresponds to the bilaterian head-forming region.

We show here that in the sea anemone Nematostella vectensis genes regulating bilaterian head development are expressed at the larval aboral pole and that a key anterior developmental gene, six3/6, controls the development of the aboral pole. These findings support the hypothesis that the anterior, head-forming, region of bilaterians and the aboral region of cnidarians derived from the same domain of their last common ancestor and are therefore homologues.