Cell Adhesion Receptors and Nuclear Receptors Are Highly Conserved from the Lowest Metazoa (Marine Sponges) to Vertebrates

The shift from unicellular life to multicellular, integrated organisms has been accompanied by the acquisition of adhesion proteins/receptors. Recently we succeeded to clone some genes coding for such proteins from the lowest multicellular animals, the marine sponges (model: the siliceous sponge Geodia cydonium). G. cydonium contains e.g. several lectins; cDNAs for two of them were 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, the cDNA coding for a receptor tyrosine kinase of class II was isolated and characterized. The deduced aa sequence shows two characteristic domains; (i) the tyrosine kinase domain and (ii) an immunoglobulin-like domain. The latter part displays 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. Considering the fact that during embryogenesis of sponges a typical anteroposterior organization pattern is seen a home-otic-organ-like transformation has been postulated. The subsequent search for genes provided with the homeodomain-like sequence was successful. These data support the view that the kingdom Animalia is of monophyletic origin.