Fig. 7

Model of sequence cooption revealing the existence of piRNA production heterogeneity within a piRNA cluster. A Maternally inherited piRNAs maintain active germline piRNA cluster at each generation (left), whereas paternal gametes transmit their DNA but no piRNAs to their progenies (right). B When a newly horizontally transferred TE inserts into a piRNA cluster, it is not efficiently targeted at the first generation (G1) by the maternal piRNA pool, even if the region is transcribed, resulting in a non-piRNA cluster conversion. This context is modeled in this study by the paternally inherited P-derived transgene, P(lArB) (red arrow), having a size closed to some autonomous TE but lacking complementary piRNAs. However, although paternally inherited and lacking homologous piRNAs, shorter sequences can be efficiently converted in G1, like T3 (brown arrow). C Four generations (G4) can be sufficient to convert full-length TE, leading to new piRNA production. This conversion can lead to the synthesis of piRNAs with different piRNA profiles throughout the locus. Sense and antisense schematic piRNA profiles are extrapolated from this study, Marie et al., and Asif-Laidin et al. [26, 35]. Arrows symbolize TE fragments or repeats found in piRNA clusters, red and blue are for the sense and antisense piRNAs, and darker colors represent the “new” piRNAs, i.e., identified in a given generation (G1 or G4) and absent in the parents