Origin of insulin receptor-like tyrosine kinases in marine sponges

One autapomorphic character restricted to all Metazoa including Porifera [s ponges] is the existence of transmembrane receptor tyrosine kinases (RTKs). In this study we screened for molecules from one subfamily within the supe rfamily of the insulin receptors. The subfamily includes the insulin recept ors (InsR), the insulin-like growth factor I receptors, and the InsR-relate d receptors-all found in vertebrates-as well as the InsR-homolog from Droso phila melanogaster. cDNAs encoding putative InsRs were isolated from the he xactinellid sponge Aphrocallistes vastus, the demosponge Suberites domuncul a, and the calcareous sponge Sycon raphanus. Phylogenetic analyses of the c atalytic domains of the putative RTKs showed that the sponge polypeptides m ust be grouped with the InsRs. The relationships revealed that all sponge s equences fall into one branch of this group, whereas related sequences from mammals (human, mouse, and rat), insects and molluscs, and polypeptides fr om one cephalochordate, fall together into a second branch. We have conclud ed that (i) the InsR-like molecules evolved in sponges prior to the "Cambri an Explosion" and contributed to the rapid appearance of the higher metazoa n phyla; (ii) the sponges constitute a monophyletic taxon, and (iii) epider mal growth factor (EGF)-like domains are present in sponges, which allows t he insertion of this domain into potential receptor and matrix molecules.