The piRNA pathway silences retrotransposons and hence maintains genome integrity in the germline. Several components of the piRNA pathway localise to a structure called the nuage, which has been detected in many animal germlines, including mouse testes and Drosophila oocytes. Now, Ai Khim Lim, Barbara Knowles and colleagues show that a nuage-like structure can be found in mouse oocytes (). They report that the nuage proteins mouse vasa homologue (MVH), Piwi-like 2 (PIWIL2/MILI) and tudor domain-containing 9 (TDRD9) transiently colocalise to a nuage-like structure in mouse oocytes shortly after birth. Furthermore, they report, the nuage protein GASZ, which is functionally but not structurally linked to the nuage in testes, is also present in cytoplasmic granules in oocytes. Using mutant mice, the authors demonstrate that the nuage genes Mvh, Mili and Gasz control retrotransposon repression through the piRNA pathway. Importantly, however, they find that these null-mutant females, unlike their male counterparts, are fertile, thus highlighting that retrotransposon activation and sterility are uncoupled in female mice.
