Figure 3.

Transgenerational effects of piRNAs in Drosophila. (A) Insertion of a LacZ sequence into a subtelomeric piRNA cluster leads to production of piRNAs corresponding to the LacZ sequence. These piRNAs can silence the expression of a LacZ reporter gene in trans. Maternal inheritance of the lacZ-containing piRNA cluster leads to silencing of the reporter, while paternal inheritance does not. (B) Scheme of hybrid dysgenesis. The presence of a potentially active TE in the genome correlates with expression of piRNAs targeting this element derived from piRNA clusters that contain the TE sequence. piRNAs targeting the element are transmitted from the maternal germline into the embryo and silence the TE in the progeny, keeping it fertile. If the TE is present in the paternal but not the maternal genome, no piRNAs targeting the TE are deposited into the embryo. The presence of the TE sequences in the paternally inherited piRNA cluster is not sufficient to protect the progeny against sterility. (C) The inducer locus I gives rise to piRNAs, which are transmitted into the next generation through the female germline. The maternally transmitted piRNAs from the I locus suffice to convert the recipient locus R, which did not produce piRNAs, into a piRNA-producing locus R*. If a female fly containing R* is crossed to a male containing the unproductive R locus (R2), the maternally deposited piRNAs can again switch R2 into R2*. Conversion to a piRNA-producing locus is stable over many generations.