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Fig. 1. Generation of germline mosaics. (A) Each germline cyst arises from a single cystoblast through four consecutive rounds of mitosis. Incomplete cytokinesis after each mitosis results in a stereotyped pattern of cytoplasmic bridges between the 16 cells of the cyst. Either of the two cells with four ring canals will become the oocyte. Recombination in the germline stem cell divisions generates clonal germline cysts (not shown), while mitotic recombination during the first division of a heterozygous cystoblast results in a mosaic cyst consisting of eight wild-type cells (green) and eight mutant cells (blue). In such mosaics, the oocyte nucleus can be either wild type (left) or mutant (right). (B) Generation of mosaics with the nuclear GFP marker. Induction of the Flp recombinase (not shown) in heterozygous cells (left, shown after DNA replication but before mitosis) mediates mitotic recombination at FRT sites (triangles), resulting in homozygous daughter cells. grk^2B6/grk^2B6 cells are marked by the lack of GFP expression. (C) In the lacO/GFP-LacI system, the starting heterozygous cells contain the lacO transgene in cis to the grk^2B6 mutation, as well as the GFP-LacI transgene. All cells exhibit nuclear GFP-LacI fluorescence, and discrete fluorescent foci are visible in the nuclei of cells with the lacO transgene. Heterozygous females exhibit a single focus of GFP in the oocyte nucleus and multiple foci in the polyploid nurse cells. Homozygous wild-type daughter cells lack these foci, while grk^2B6/grk^2B6 oocyte nuclei exhibit two foci.

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