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Metazoan genomes are largely composed of repeated sequences, including transposable elements (TEs). TEs are DNA sequences that can move from one genomic locus to another. Their mobilization causes genomic instabilities, which can lead to pathologies. In animal gonads, TEs expression is restricted by a specific pathway involving small non-coding RNAs called piRNAs (PIWI-interacting RNAs). These piRNAs target TEs by sequence complementarity, preventing their expression and mobilization.

In Drosophila melanogaster, the piRNA pathway is active in adult female gonads: in the germ cells (GCs), but also in somatic gonadal cells (SGCs) surrounding GCs. Some TEs which are abnormally reactivated in SGCs can infect and invade the germline genome. Therefore, activation of the somatic piRNA pathway is also essential for genome protection of the offspring.

The formation of the gonads starts at the embryonic stage. Little is known about the activation of the piRNA pathway and the establishment of TEs repression during development. In our study, we investigated the developmental window during which the somatic piRNA pathway becomes active in the gonads. We monitored the establishment of the repression of TEs, such as 412, known to be silenced by the flamenco piRNA cluster, the major locus involved in piRNA production in SGCs. Our results reveal that flamenco and the piRNA pathway actors start to be expressed in SGCs during embryonic gonad formation. Surprisingly TEs are expressed in embryonic SGcs, and progressively repressed later in development, from the larval stage onwards. These results suggest that the somatic piRNA pathway is initiated during embryonic gonad formation and becomes functional during larval stage.