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Fig. 11 Schematics depicting the PGC to germline cysts with germline stem cell transition. (A) In wild-type gonads, individual PGCs cluster near one another and undergo dramatic nuclear rearrangements, possibly reflecting the onset of differentiation into germline stem or progenitor cells. During the transition step, the cell boundaries (pink) are not apparent as nanos2 (yellow) becomes detectable as discrete puncta with low and high regions of expression. Incomplete cytokinesis generates progenitors with round nuclei connected by intracellular bridges. Most cells express low levels of nanos2, whereas a few express high levels of nanos2, the likely GSCs. (B) In dazl mutants the first steps are intact. Mutant germ cells enter the transition state and express low levels of nanos2, but not high levels. As gonad development ensues, only individual GCs are detected in mutants and no GSCs are specified. It is unclear whether GCs revert to the pre-transition state or whether they are lost; however, blocking cell death does not prevent the germline loss or sterility of dazl mutants. Thus, Dazl is essential for germline cyst formation and GSC specification, either directly or indirectly.