ON THE MONOPHYLETIC EVOLUTION OF THE METAZOA

1. The shift from unicellular life to multicellular, integrated organi sms has been accompanied by the acquisition of adhesion proteins. Rece ntly we succeeded in cloning some genes coding for such proteins from the lowest multicellular animals, the marine sponges (model: the silic eous sponge Geodia cydonium). 2. G. cydonium contains several lectins and cDNA for two of them (termed LECT-1 and LECT-2) was cloned. Both l ectins 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 s equence shows two characteristic domains: i) the tyrosine kinase domai n and ii) an immunoglobulin-like domain. The latter part shows high ho mology to the vertebrate type immunoglobulin domain. This result, toge ther with the lectin data, demonstrates that binding domains of such a dhesion proteins are not recent achievements of higher animals but exi st already in animals (sponges) which have diverged from other organis ms about 800 million years ago. 3. Considering the fact that during em bryogenesis 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 sequenc e was successful. The deduced amino acid sequence of G. cydonium showe d high homology to chicken and to the Antennapedia sequence from Droso phila melanogaster. 4. These data support the view that the kingdom An imalia is of monophyletic origin.