Molecular origin of mesoderm : conservation of T-box genes in non-bilaterian animals

Several members of the T-box gene family were characterized from cnidarians, ctenophores, sponges and placozoans, which are considered as the four metazoan phyla at the basis of animal evolution leading to bilaterians. T-box genes code for transcription factors with high sequence similarity in a region of about 180 amino acids called T-box domain, which is involved in DNA binding. T-box genes have important roles in many developmental processes such as mesoderm formation, antero-posterior axis formation, muscle and limb formation. Thus, T-box genes have been considered key genes to study developmental processes linked to evolutionary aspects. In this study it is shown that the four extant nonbilaterian phyla have members of well-defined T-box gene subfamilies such as Brachyury or Tbx2/3. These are clear homologs of higher evolved bilaterian genes, suggesting the presence of a complex T-box gene family already in the hypothetical common ancestor of all metazoans (Urmetazoa). The data collected in this study were also used to test the phylogenetic relationship of the four basal metazoan phyla in relation to bilaterian animals. Molecular phylogeny has redefined the evolutionary tree, introducing the ecdysozoa and lophotrochozoa as new clades of protostomes, but did not solve the relationship and the branching order of many phyla, especially of the four phyla at the base of animal evolution. In this study most of the investigations were done to clarify the position of the placozoan Trichoplax adhaerens, which is an enigmatic organism that has been interpreted either as a derived cnidarian or as a primitive non-bilaterian within its own phylum named Placozoa. The phylogenetic analyses reported here confirm that Trichoplax forms an own phylum, but does not really improve the knowledge about its phylogenetic position. An important improvement for the investigation of the biology of Trichoplax was achieved by adapting in situ hybridisation methods. Experiments with T-box genes such as Brachyury and Tbx2/3, homeobox genes such as Not and Trox-2, and additional reference genes suggest that Trichoplax has more than the four cell types described in the literature and that the process of regeneration could be regulated by a homeobox gene of the Not class.