The shift from unicellular life to multicellular, integrated organisms has been accompanied by the acquisition of adhesion proteins. Recently we succeeded in cloning some genes coding for such proteins from the lowest multicellular animals, the marine sponges (model: the siliceous sponge Geodia cydonium). 2. G. cydonium contains several lectins and cDNA for two of them (termed LECT-1 and LECT-2) was cloned. Both lectins have a framework sequence of 38 conserved amino acids which are characteristic for the carbohydrate-binding site of vertebrate S-type lectins. Next, we have isolated and characterized a cDNA coding for a receptor tyrosine kinase of class II (GCTK). The deduced amino acid sequence shows two characteristic domains: i) the tyrosine kinase domain and ii) an immunoglobulin-like domain. The latter part shows high homology to the vertebrate type immunoglobulin domain. This result, together with the lectin data, demonstrates that binding domains of such adhesion proteins are not recent achievements of higher animals but exist already in animals (sponges) which have diverged from other organisms about 800 million years ago. 3. Considering the fact that during embryogenesis of sponges a typical anteroposterior organization pattern is seen, a "homeotic" organ-like transformation has been postulated. The subsequent search for genes provided with the homeodomain sequence was successful. The deduced amino acid sequence of G. cydonium showed high homology to chicken and to the Antennapedia sequence from Drosophila melanogaster. 4. These data support the view that the kingdom Animalia is of monophyletic origin.
