Bauplan of Urmetazoa:: Basis for genetic complexity of Metazoa

Sponges were first grouped to the animal-plants or plant-animals then to the Zoophyta or Mesozoa and finally to the Parazoa. Only after the application of molecular biological techniques was it possible to place the Porifera monophyletically with the other metazoan phyla, justifying a unification of all multicellular animals to only one kingdom, the Metazoa. The first strong support came from the discovery that cell-cell and cell-matrix adhesion molecules that were cloned from sponges and were subsequently expressed share a high DNA sequence and protein function similarity with the corresponding molecules of other metazoans. Besides these evolutionary novelties for Metazoa, sponges also have morphogens and transcription factors in common with other metazoan phyla. Surprisingly, even those elements exist in Porifera, which are characteristic for pattern and axis formation. Recent studies showed that epithelial layers of sponges are sealed against the extracellular milieu through tight-junction proteins. The cell culture system from sponges, the primmorphs, was suitable for understanding morphogenetic events. Finally, stem cell marker genes were isolated, which underscored that sponge cells have the capacity to differentiate. In the relatively short period of time, approximately 200 million years, the basic pathways had to be established that allowed the transition for multicellular organisms to a colonial system through the formation of adhesion molecules; based on the development of a complex immune system and the apoptotic machinery of an integrated system, the Urmetazoa, which evolved approximately 800 million years ago, could be reached. Hence, the Bauplan of the hypothetical Urmetazoa can now be constructed according to genomic regulatory systems similar to those found in higher Metazoa. These data caused a paradigmatic change; the Porifera are complex and simple but by far not primitive.