Figure 1. Paramutation of the BX2 locus involves maternally inherited T-1 piRNAs. (A) Whereas the T-1 transgene cluster produces piRNAs (small red dashes), the BX2 transgene cluster does not; these distinct properties are completely stable over generations. When T-1 females are crossed to BX2 males (G0), the female progeny (G1) that inherited the BX2 chromosome from fathers and T-1-derived piRNAs from the mother (but not the T-1 chromosome) start to zygotically produce high levels of BX2-derived piRNAs. The inactive (blue) to active (red) state transition of the BX2 locus is noted with an asterisk (BX2*). This so-called paramutation can be further maternally inherited in the next generations (Gn). (B) Similarly as in (A), maternal inheritance of BX2*-derived piRNAs triggers the state transition of an inactive BX2 loci in G1, associated to zygotic production of piRNAs. This second-order paramutation is noted as BX2*² and can be further maternally inherited in the next generations (Gn). The seven repeats of the P{lacW} transgene in the T-1 and BX2 loci are represented by blue or red arrowheads, depending on the states of the loci (active in red, inactive in blue).