Figure 1. piRNAs control mobile elements in the Drosophila germline.
piRNAs are generated from specific loci throughout the Drosophila germline genome. Two examples are depicted. The Flamenco and X-TAS loci are located in heterochromatic regions on the X chromosome. Hundreds of distinct piRNAs are produced from each of these loci and correspond to mobile element repeats dispersed throughout the genome. According to the current model, piRNAs associate with Piwi proteins in the germline and serve as guides that lead to cleavage of transposon targets. Flamenco is known to control expression of the gypsy, Idefix, and ZAM retroelements, and X-TAS has been linked to the control of P-elements.