Fig. 1. Cell cycle transitions in epithelial follicle cells. (A) Follicle cells in Drosophila oogenesis undergo two cell cycle transitions: from a mitotic cell cycle to an endocycle and from endocycle to amplification (A, parts I-III, drawing of the stages of oogenesis). From the germarium (g) to stage (st) 6, somatically derived follicle cells undergo mitotic cycles, which are not synchronized (A, part I). At stage 7 they switch to endocycles. From stage 7 to stage 10A, these cells undergo three rounds of endoreplication (A, part II) and thereafter switch to the localized replication pattern characteristic of chorion gene amplification (A, part III). These three replication patterns are observed in BrdU incorporation analysis (B). The staining of mitotic markers Cyclin B (CycB) (red arrows), Phospho-Histone 3 (PH3) (green arrows) (C) and stg (6.4) promoter construct (D) show expression in mitotic follicle cells, whereas Fzr enhancer trap line, fzrG0326 (ß-gal expression, red arrow) shows Fzr expression in endocycling cells (E). Green, PH3; red, CycB (C); Anti-ß-gal (D,E); blue, DAPI (C-E).