ORIGIN OF METAZOA - SPONGES AS LIVING FOSSILS

The phylogenetic position of the phylum Porifera (sponges) is at the b ase of the kingdom Metazoa. During the past few years not only rDNA se quences but - and this was a major advance - even cDNAs/genes have bee n isolated and characterized from sponges, especially from the marine demosponge Geodia cydonium, which code for proteins. The analyses of t heir deduced amino acid sequences allowed a molecular biological appro ach to solve the problem of monophyly of Metazoa. Molecules of the ext racellular matrix/basal lamina, with the integrin receptor, fibronecti n, and galectin as prominent examples, cell-surface receptors (tyrosin e kinase receptor), elements of sensory systems (crystallin, metabotro pic glutamate receptor), and homologs/modules of an immune system (imm unoglobulin like molecules, scavenger receptor cysteine-rich, and shor t consensus repeats, rhesus system) classify the Porifera as true Meta zoa. As living fossils, provided with simple, primordial molecules all owing cell-cell and cell-matrix adhesion as well as processes of signa l transduction as known in a more complex manner from higher Metazoa, they also show peculiarities not known in other metazoan phyla. Tissue s of sponges are rich in telomerase activity, suggesting a high plasti city in the determination of cell lineages. It is concluded that molec ular biological studies with sponges as model will not only help to un derstand the evolution of Protoctista to Metazoa but also the complex, hierarchial regulatory network of cells in higher Metazoa.