SCHIERWATER, B; JAKOB, W; SAGASSER, S; DELLAPORTA, S; TiHo Hannover; TiHo Hannover; TiHo Hannover; Yale University: Trichoplax adhaerens and B�tschlis� �urmetazoan� model system

A more than a century old debate has been focusing on the origin of the Metazoa and the hypothetical �urmetazoan� bauplan. A basal position for the morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, was widely accepted for almost a century. The limited number of only four somatic cell types, the smallest metazoan genome, and the lack of any foot or head structures, of any anterior-posterior organization, any kind of organs, and of both a basal lamina and an extracellular matrix (ECM) placed Trichoplax in a basal and isolated position relative to all other metazoan phyla. B�tschlis� placula hypothesis of the �urmetazoon� was based on this view and provided a hypothetical scenario for both, the evolution of diploblast and triploblast bauplans.. We have been analysing (i) the sum of morphological data, (ii) the molecular sequence data from two mitochondrial genes, COII and COIII, (iii) the molecular sequence data from two regulatory nuclear genes, Cnox-2 and the PaxB gene, (iv) the secondary structure of the 16S rRNA molecule, and (v) the putative cluster organization of Hox genes. The evidence strongly supports the view that Placozoa are basal relative to all other recent diploblast phyla. Furthermore, the comparison of Hox/ParaHox gene expression patterns provide an intriguing hypopthetical scenario for B�tschlis� original placula hypothesis and thus the hypothetical �urmetazoan� bauplan.