RNAi protects the Caenorhabditis elegans germline against transposition

The availability of complete genome sequences has revealed that genomes contain numerous mobile genetic elements. The activity of these transposons results in genome instability. In the nematode Caenorhabditis elegans, transposition is silenced in the germline, protecting the genome from heritable defects that are caused by transposon jumps. The discovery of RNA interference (RNAi) has greatly increased our understanding of this genome-defense mechanism because transposon silencing and RNAi share common factors. RNAi is the post-transcriptional silencing of a gene in response to double-stranded RNA (dsRNA), which is processed into small interfering RNAs (siRNAs) that mediate sequence-specific RNA degradation. Indeed, transposon-derived dsRNA and siRNAs are detected in vivo and are capable of inducing silencing. In addition, many new genes have been identified that are required for the silencing of transposons. In this article, we discuss our current model for transposon silencing and address several unanswered questions because many aspects of this protection mechanism in C. elegans might be present in a broad range of organisms.