The Evolutionary Origin of the Gut

The inner-most, gut-forming endoderm has always been considered as evolutionary related between cnidarians and other animals. In their study, Technau and colleagues have now tested this hypothesis by tracing the embryonic origin of digestive enzyme-producing cells as well as their developmental regulator genes typical of the gut and pancreas in a sea anemone. The authors show that in sea anemones, against all previous beliefs, digestive enzyme- and insulin-producing gland cells do not develop from endoderm but from the ectodermal part of the mouth, the pharynx. “I was puzzled when I first saw that all endoderm derivatives of sea anemones are totally devoid of digestive gland cells. That was not what is taught in biology textbooks” explains Patrick Steinmetz, who contributed most of the experiments and is now a group leader at the University of Bergen in Norway.

“The results completely change the way we think of the origin of germ layers. It means that ‘endoderm’ in sea anemones and vertebrates, although they are called the same, are actually not evolutionary related” adds Ulrich Technau. If the mouth ectoderm of the sea anemone and not the endoderm corresponds to the vertebrate gut and pancreas, then what is the vertebrate correlate of the sea anemone endoderm? When Steinmetz and Technau dwelled deeper into this question, they found strong similarities between the cnidarian endoderm and the intermediate mesoderm layer: both share a large number of regulatory genes, and both give rise to similar cell types such as muscle or gonad cells. The sea anemone thus shows a clear correlate of mesoderm, but not in an intermediate position as found in three-layered animals. Positioning, and not novel emergence, of tissue in-between the gut and skin was thus the key event that led to the evolution of three-layered animals.

“An overwhelming majority of animals nowadays develop three germ layers, and we have taken a big step towards the understanding of one of the most crucial events underlying this evolutionary success story” concludes Steinmetz.

Contacts and sources:
Univ.-Prof. Dr. Ulrich Technau
Department for Molecular Evolution and Development
Centre for Organismal Systems Biology
Faculty of Life Sciences
University of Vienna

Steinmetz, P.R.H., Aman, A., Kraus, J.E.M., Technau, U. Gut-like ectodermal tissue in a sea anemone challenges germ layer homology. Nature Ecology & Evolution DOI: 10.1038/s41559-017-0285-5.