Fig. 3. The piRNA pathway is required for germline development. (A) piwi is required for the self-renewing division of germline stem cells during oogenesis. DAPI-stained images of 0- to 1-day-old adult ovarioles from wild-type (WT; +/+) and piwi2 Drosophila. WT ovarioles contain a long string of developing egg chambers produced through continuous stem cell division. By contrast, piwi mutant ovarioles typically contain only two egg chambers, derived through stem cell differentiation and loss. Ge, germarium; S1, stage 1 egg chamber [from Cox et al. (Cox et al., 1998)]. (B) Increased DNA damage in the germline cells of aub mutant ovaries. Drosophila ovaries immunostained to reveal the phosphorylated form of histone H2Av (-H2Av, red) and DNA (blue). -H2Av accumulates near double-strand break (DSB) sites. In WT ovaries, -H2Av foci are restricted to region 2 of the germarium, where meiotic DSBs form. In aub mutant ovaries, -H2Av foci accumulate in cells within the germarium and persist and increase in intensity only in the germline as cysts bud from the germarium to form egg chambers [from Klattenhoff et al. (Klattenhoff et al., 2007)]. (C) Miwi2 mutation depletes germ cell lineages in mouse testes. Hematoxylin and Eosin (HE) staining of WT and Miwi2 mutant adult testes shows germline degeneration in Miwi2 mutant mouse testes [from Carmell et al. (Carmell et al., 2007)]. (D) Ziwi is necessary for the maintenance of the zebrafish germline. HE staining shows that ziwi mutants have germ-cell-depleted testes, as compared with WT [from Houwing et al. (Houwing et al., 2007)]. (A) Reprinted with permission from Cold Spring Harbor Laboratory Press; (B,C,D) reprinted with permission from Elsevier.