Distribution of vkr genes in metazoan phyla. Vkr genes were definitely identified in Arthropoda (27 insect species), Platyhelminthes (trematode and cestodes parasites), and in Mollusca, Annelida and Echinodermata (at least in one species for each phylum). Additionally, putative VKR sequences have been detected in the genomes of many other arthropods, in the mollusc Biomphalaria glabrata, in the annelid Helobdella robusta and in the echinoderm Paracentrotus lividus. The presence of VKR has been confirmed in the cnidarian N. vectensis but not found in Hydra (H. magnipapillata and H. vulgaris) genomes. The “unclassified” TK (UTK12)  and the RTKS kinase  found respectively in the choanoflagellates Monosiga brevicollis and Salpingoeca rosetta possess an architecture similar to VKR proteins. However, no vkr gene was found in the poriferan Amphimedon queenslandica. Concerning Ctenophora, Acoela, Rotifera, Ectoprocta, Brachiopoda phyla, genomic data are currently not sufficient to assess the presence or not of vkr genes in these organisms. No vkr genes were found in Chordata (vertebrates) nor in Nematoda (worms).