Maternally transmitted factors (possibly small RNAs), and not the genomic composition, are responsible for the absence of P-element ping-pong in replicate 2. (A) Crossing scheme for testing the influence of maternal piRNAs. We performed reciprocal crosses among replicates with (R1, R4) and without (R2) ping-pong signature for the P-element. Note that in the F1 offspring of the reciprocal crosses (e.g., R1 × R2 vs. R2 × R1), the genomic background is largely identical, whereas the composition of the maternally deposited piRNAs differs. The flies for this experiment were sampled around generation 70 from the experimental populations. (B) Ping-pong signatures for the P-element in all three replicates (R1, R2, R4) and in the F1 offspring of the reciprocal crosses among the replicates (e.g., R1 × R2: R1-female × R2-male). Small RNA was extracted from whole female flies, and three subreplicates (right panel) were used for each sample. (C) Distribution of piRNAs (23–29 nt) in the three replicates and the F1 offspring. (D) Distribution of siRNAS (20–22 nt) in the three replicates and the F1 offspring.
