The Drosophila endocycle is controlled by Cyclin E and lacks a checkpoint ensuring S-phase completion

Early during Drosophila oogenesis the 16 interconnected cells of each germ-line cyst choose between two alternative fates. The single future oocyte enters meiosis, arrests, and becomes transcriptionally quiescent. The remaining 15 cells initiate a series of polyploid cell cycles to prepare for their role as nurse cells. Like many other polyploid and polytene cells, during nurse cell growth the major satellite DNAs become highly under-represented by a mechanism that has remained obscure. We implicate the cell-cycle regulator cyclin E in DNA under-representation by identifying a hypomorphic, female sterile cycE mutation, cycE01672, that increases the amount of satellite DNA propagated in nurse cells. In mutant but not wild-type endomitotic nurse cells, "late S" patterns of bromodeoxyuridine incorporation are observed similar to those in mitotic cells. CycE protein still cycles in cycE01672 germ-line cysts but at reduced levels, and it is found throughout a longer fraction of the cell cycle. Our experiments support the view that oscillating levels of CycE control the polyploid S phase. Moreover, they indicate that a checkpoint linking the presence of unreplicated DNA to the CycE oscillator is lacking, leading to incomplete replication of late-replicating sequences such as satellite DNAs. Unexpectedly, two to three of the 16 cells in cycE01672 cysts frequently differentiate as oocytes, implicating cell-cycle programming in oocyte determination.