Throughout the eukaryotic lineage, small RNA silencing pathways protect the genome against the deleterious influence of selfish genetic elements such as transposons. In animals an elaborate small RNA pathway centered on PIWI proteins and their interacting piRNAs silences transposons within the germline. In contrast to other small RNA silencing pathways, we lack a mechanistic understanding of this genome defense system. However, genetic and molecular studies have uncovered a fascinating conceptual framework for this pathway that is conserved from sponges to mammals. We discuss our current understanding of the piRNA pathway in Drosophila with an emphasis on origin and biogenesis of piRNAs.
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