Endogenous siRNAs derived from transposons and mRNAs in Drosophila somatic cells

Small interfering RNAs ( siRNAs) direct RNA interference ( RNAi) in eukaryotes. In flies, somatic cells produce siRNAs from exogenous double- stranded RNA ( dsRNA) as a defense against viral infection. We identified endogenous siRNAs ( endo- siRNAs), 21 nucleotides in length, that correspond to transposons and heterochromatic sequences in the somatic cells of Drosophila melanogaster. We also detected endo- siRNAs complementary to messenger RNAs ( mRNAs); these siRNAs disproportionately mapped to the complementary regions of overlapping mRNAs predicted to form double- stranded RNA in vivo. Normal accumulation of somatic endo- siRNAs requires the siRNA- generating ribonuclease Dicer- 2 and the RNAi effector protein Argonaute2 ( Ago2). We propose that endo- siRNAs generated by the fly RNAi pathway silence selfish genetic elements in the soma, much as Piwi- interacting RNAs do in the germ line.