During Drosophila oogenesis, transposable element (TE) repression involves the 
Piwi-interacting RNA (piRNA) pathway which ensures genome integrity for the next 
generation. We developed a transgenic model to study repression of the Idefix 
retrotransposon in the germline. Using a candidate gene KD-approach, we 
identified differences in the spatio-temporal requirements of the piRNA pathway 
components for piRNA-mediated silencing. Some of them (Aub, Vasa, Spn-E) are 
necessary in very early stages of oogenesis within the germarium and appear to 
be less important for efficient TE silencing thereafter. Others (Piwi, Ago3, 
Mael) are required at all stages of oogenesis. Moreover, during early oogenesis, 
in the dividing cysts within the germarium, Idefix anti-sense transgenes escape 
host control, and this is associated with very low piwi expression. Silencing of 
P-element-based transgenes is also strongly weakened in these cysts. This 
region, termed the 'Piwiless pocket' or Pilp, may ensure that new TE insertions 
occur and are transmitted to the next generation, thereby contributing to genome 
dynamics. In contrast, piRNA-mediated silencing is strong in germline stem cells 
in which TE mobilization is tightly repressed ensuring the continued production 
of viable germline cysts.