Ancient divergence of animal protein tyrosine kinase genes demonstrated by a gene family tree including choanoflagellate genes

Animal-specific gene families involved in cell–cell communication and developmental control comprise many subfamilies with distinct domain structures and functions. They diverged by subfamily-generating duplications and domain shufflings before the parazoan–eumetazoan split. Here, we have cloned 40 PTK cDNAs from choanoflagellates, Monosiga ovata, Stephanoeca diplocostata and Codosiga gracilis, the closest relatives to animals. A phylogeny-based analysis of PTKs revealed that 40 out of 47 subfamilies analyzed have unique domain structures and are possibly generated independently in animal and choanoflagellate lineages by domain shufflings. Seven cytoplasmic subfamilies showed divergence before the animal–choanoflagellate split originated by both duplications and shufflings.