Conserved association of Argonaute 1 and 2 proteins with miRNA and siRNA pathways throughout insect evolution, from cockroaches to flies

Background: Argonaute proteins are key in RNA silencing. In Drosophila melanogaster, the five proteins of the Argonaute family participate in the pathways and mechanisms mediated by three types of small RNAs: piRNAs, miRNAs, and siRNAs. Two Argonaute proteins, Argonaute 1 (Ago1) and Argonaute 2 (Ago2), are associated with miRNA and siRNA mechanisms, which are the most thoroughly studied. The available data points to a sorting specialization of Ago1 for miRNAs and Ago2 for siRNAs. However, this has been demonstrated only in D. melanogaster, one of the most modified insects, which emerged some 100 million years ago. Thus, an important question is whether this association of Ago1 with miRNAs and Ago2 with siRNAs occurs generally in insects, or was a specific innovation in higher flies. Methods: We addressed this question by using RNAi approaches and studying Ago1 and Ago2 functions in the German cockroach, Blattella germanica, a much less modified insect that emerged some 320 million years ago. Results: The results showed that B. germanica does preferentially use Ago1 in the miRNA pathway, but can also use Ago2 in some cases. Conversely, Ago2 operates in the RNAi, in siRNA sorting, whereas Ago1 seems to have no relevant role in this process. Conclusions and general significance: These basic associations are equivalent to those observed in D. melanogaster, implying that they have been evolutionary conserved from at least cockroach to flies, and possibly stem from the last common ancestor of extant insects.