Distribution of RdRP in eukaryotes. Although RdRPs are present in many plants, fungi and protists (a selection is included in this tree), of the Metazoa, only Caenorhabditid nematodes and a chordate Branchiostoma are found to carry RdRP genes. Plant and protist RdRPs cluster together with very high support, while fungus and animal RdRPs comprise distinct clusters. Caenorhabditid RdRPs are represented by the three C. elegans paralogs RRF-1/3 and Ego-1. Species names of the organisms shown in this tree are as follows: animals, Branchiostoma floridae; fungi, Coccidioides immitis, S. pombe, Neurospora crassa and Aspergillus terreus; plants,Hordeum vulgare, Arabidopsis thaliana, Nicotiana tabacum and Solanum lycopersicum; protists, Dictyostelium discoideum and Tetrahymena thermophila.