Fig. 1. Switch of cell-cycle programs and the Cut expression pattern in follicle cells. (A) During Drosophila oogenesis, somatically derived follicle cells undergo two cell-cycle switches: (1) the mitotic cycle to endocycle switch and (2) the endocycle to gene-amplification switch. From the germarium (G) to stage (S) 6, follicle cells undergo unsynchronized mitotic cycles. During stages 7 to 10A, these cells go through three rounds of endoreplication and thereafter switch to a localized amplification pattern characteristic of chorion gene amplification. (B) Cut expression in follicle cells. Expression (shown in red) began in region 2b of the germarium. It persisted in all follicle cells, including the polar cells and the interfollicular stalk cells, until about stage 6 and diminished afterwards, concurrent with the first cell-cycle switch. PH3 (shown in green) was used to mark the M phase of the mitotic cycle. Between stages 7 and 10A of oogenesis, Cut expression ceased in all follicle cells except the polar cells. At about stage 10B, Cut expression resumed in the columnar follicle cells that surround the oocyte. DAPI (in blue) was used to reveal the nuclei.