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

In the present review we summarize sequence data obtained from cloning of sponge receptor tyrosine kinases [RTK]. The cDNA sequences were mainly obtained from the marine sponge Geodia cydonium. RTKs (i) with immunoglobulin [Ig]-like domains in the extracellular region, (ii) of the type of insulin-like receptors, as well as (iii) RTKs with one extracellular speract domain, have been identified. The analyses revealed that the RTK genes are constructed 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 sequences, together form a second branch. It is concluded that the RTK molecules have evolved in sponges prior to the "Cambrian Explosion" and have contributed 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 kinases in general and RTKs in particular have only been identified in Metazoa, they are, as a group qualified, to be considered as an autapomorphic character of all metazoan phyla.