A Naïve RNA Sampling Core Enables Adaptive piRNA Specificity Against Transposable Elements [CLIP]

How piRNA-mediated genome defense achieves specificity against transposons while sampling a complex transcriptome has remained unresolved. Here we show that piRNA biogenesis operates through pervasive, non-specific sampling of cytoplasmic RNAs, with specificity imposed by tissue-specific molecular modules that exploit intrinsic vulnerabilities of transposons. In Drosophila somatic cells, the specificity factor Yb steers basal processing towards uridine-rich RNAs—automatically capturing antisense retrotransposon transcripts due to their intrinsically adenosine-biased genomes. In germline cells lacking Yb, basal sampling generates naïve piRNAs loaded into catalytically active Argonaute proteins, which trigger autocatalytic ping-pong amplification upon encountering complementary targets. In both contexts, transposon mobility facilitates the production of antisense RNAs that enable either biased processing or amplification. Thus, piRNA clusters, long associated with pathway specificity, act as sources of transposon antisense sequences, while specificity arises from layering distinct molecular mechanisms onto a shared foundation of indiscriminate transcript sampling, enabling robust and adaptable genome defense without predefined templates. Overall design: To define transcriptome-wide RNA binding sites of the piRNA-pathway factor Yb in Drosophila melanogaster ovarian somatic cells (OSCs), we performed UV crosslinking and immunoprecipitation followed by sequencing (CLIP-seq) using a monoclonal anti-Yb antibody. OSCs were washed with PBS and UV-crosslinked on ice at 254 nm (200 mJ/cm² twice with 90° rotation). Crosslinked cells were lysed, clarified, and treated with RNase I to fragment RNA, and Yb–RNA complexes were isolated by immunoprecipitation on antibody-coupled magnetic beads under stringent high-salt wash conditions. Protein–RNA complexes were resolved by SDS–PAGE, transferred to nitrocellulose, excised based on radiolabeled signal, and recovered RNA was used to generate gel-based small RNA libraries for Illumina sequencing (single-end 50 bp). The experiment includes two biological replicates of Yb CLIP (CLIP_YB_Rep1, CLIP_YB_Rep2) and an input control (CLIP_INPUT). Processed data include strand-specific bigWig coverage tracks for all reads and uniquely mapped reads (sense and antisense) to enable comparison of Yb-bound RNA profiles across replicates and relative to input.