Gene structure of ago-1 and dcr-1 in Acyrthosiphon pisum . For each gene, both copies have a similar exon/intron structure. Apiago-1a and Apiago-1b have 18 exons, most of them packed in three different regions. The gene encodes a protein with an N-terminal domain of unknown function (DUF1785), a Piwi/Argonaute/Zwille domain (PAZ) and a C-terminal PIWI domain. Apidcr-1a and Apidcr-1b have 20 exons and encode a protein with an N-terminal DexD/H-box helicase domain, a dsRNA binding domain, a PAZ domain and two tandem C-terminal RNase III domains (RNase IIIa and IIIb). The RNase IIIa domain is truncated in Apidcr-1b by a deletion of 47 amino acids. The regions analyzed in this study are marked under the genes. Functional domains of the proteins are shown with approximate indication of the corresponding span in nucleotide sequence.