Eukaryotic RNA-dependent RNA polymerases (RdRPs, encoded by RDR genes) play critical roles in developmental regulation, maintenance of genome integrity, and defense against foreign nucleic acids. However, the phylogenetic relationship of RDRs remains unclear. From available genome sequences, we identified 161 putative RDR genes from 56 eukaryotes, ranging from protists to multicellular organisms, including plants, fungi and invertebrate animals, such as nematodes, lancelet and sea anemone. On the other hand, we did not detect RDR homologs in vertebrates and insects, even though RNA interference functions in these organisms. Our phylogenetic analysis of the RDR genes suggests that the eukaryotic ancestor might have had three copies, i.e. RDRα, RDRβ and RDRγ. These three ancient copies were also supported by the patterns of protein sequence motifs. Further duplication events after the divergence of major eukaryotic groups were supported by the phylogenetic analyses, including some that likely occurred before the separation of subgroups within each kingdom. We present a model for a possible evolutionary history of RDR genes in eukaryotes.
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