Meeting Abstract

S5.11  Jan. 5  Total Evidence Analysis Identifies Placozoa as Basal to Extant Metazoa SCHIERWATER, Bernd*; DESALLE, Rob; JAKOB, Wolfgang; SCHROTH, Werner; HADRYS, Heike; DELLAPORTA, Stephen; ITZ, Ecology and Evolution; Dept. of Molecular Cellular and Developmental Biology, Yale University; American Museum of Natural History; ITZ, Ecology and Evolution; ITZ, Ecology and Evolution; ITZ, Ecology and Evolution; Dept. of Molecular Cellular and Developmental Biology, Yale University bernd.schierwater@ecolevol.de

A more than a century old debate has focused on the origin of metazoan animals and the hypothetical “urmetazoan” bauplan. The morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles the intriguing model for one out of several “urmetazoan” hypotheses, the placula hypothesis of metazoan origin. If there were clear support for a basal position of Placozoa all key issues of metazoan specific inventions (including for example head-foot axis, symmetry, coelom) would find a root for unraveling their evolution. Unfortunately, the phylogenetic relationships at the base of Metazoa have been unresolved. While morphological analyses have suggested a basal position for Placozoa, molecular studies revealed different and conflicting phylogenetic scenarios. Here we analyze the sum of morphological evidence, molecular sequence data from mitochondrial and nuclear genes, the secondary structure of a mitochondrial ribosomal gene, and Hox gene expression. Together with mtDNA genome structure and sequence analyses these data provide compelling evidence that Placozoa are basal relative to all other recent metazoan phyla.