Receptor tyrosine kinase, an autapomorphic character of metazoa: Identification in marine sponges

In the present review we summarize sequence data obtained from cloning of s ponge receptor tyrosine kinases [RTK]. The cDNA sequences were mainly obtai ned from the marine sponge Geodia cydonium. RTKs (i) with immunoglobulin [I g]-like domains in the extracellular region, (ii) of the type of insulinlik e receptors, as well as (iii) RTKs with one extracellular speract domain, h ave been identified. The analyses revealed that the RTK genes are construct ed in blocks [domains], suggesting a blockwise evolution. The phylogenetic relationships of the sequences obtained revealed that all sponge sequences fall into one branch of the evolutionary tree, while related sequences from higher Metazoa, human, mouse and rat, including also invertebrate sequence s, together form a second branch, it is concluded that the RTK molecules ha ve evolved in sponges prior to the "Cambrian Explosion" and have contribute d to the rapid appearance of the higher metazoan phyla and that sponges are , as a taxon, also monophyletic. Due to the fact that protein tyrosine kina ses in general and RTKs in particular have only been identified in Metazoa, they are, as a group qualified, to be considered as an autapomorphic chara cter of all metazoan phyla.