 In C. elegans this initiation response is amplified through the synthesis of a population of ' secondary ' siRNAs during which the dicer @-@ produced initiating or ' primary ' siRNAs are used as templates . These ' secondary ' siRNAs are structurally distinct from dicer @-@ produced siRNAs and appear to be produced by an RNA @-@ dependent RNA polymerase ( RdRP ) . 
