Phylogenetic analysis of Argonaute proteins. The neighbor-joining tree is based on the alignment of the conserved PIWI domain. Argonaute proteins can be categorized into four groups, each important for a different process; the RNAi pathway, the miRNA pathway, transcriptional silencing, and amplification of the RNAi effect (secondary Argonautes). Tribolium and Drosophila lack secondary Argonautes, suggesting that the secondary Argonaute-based amplification mechanism is not conserved in these insects.
Tomoyasu et al. Genome Biology 2008 9:R10 doi:10.1186/gb-2008-9-1-r10