Fig. 1. Follicle cells in Drosophila oogenesis undergo a transition from a mitotic cell cycle to an endocycle. (A) Drawing of the stages of oogenesis. From the germarium (g) to stage (st) 6, somatically derived follicle cells undergo mitotic cell cycle. At stage 7 they switch to endocycles. From stage 7 to stage 10A, these cells undergo three rounds of endoreplication. (B) Staining of mitotic markers Cyclin B (CycB, red, red arrow) and Phospho-Histone 3 (PH3, blue, blue arrow) of two egg chambers at stages 6 and 7, showing the transition from mitotic cell cycle to endocycle at these two stages. (C) Whole-mount in situ hybridization shows that string, which encodes a Cdc25 phophatase, is expressed in follicle cells from the germarium to stage 6. string expression in follicle cells is turned off at stage 7 (arrow), suggesting that the transition from mitotic cell cycle to endocycle is regulated at the transcriptional level.
