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 quiesce nt. The remaining 15 cells initiate a series of polyploid cell cycles to prepare for their role as nurse cells. Like many other polyploid an d polytene cells, during nurse cell growth the major satellite DNAs be come highly under-represented by a mechanism that has remained obscure . We implicate the cell-cycle regulator cyclin E in DNA under-represen tation by identifying a hypomorphic, female sterile cycE mutation, cyc E(01672), that increases the amount of satellite DNA propagated in nur se cells. In mutant but not wild-type endomitotic nurse cells, ''late S'' patterns of bromodeoxyuridine incorporation are observed similar t o those in mitotic cells. CycE protein still cycles in cycE(01672) ger m-line cysts but at reduced levels, and it is found throughout a longe r fraction of the cell cycle. Our experiments support the view that os cillating levels of CycE control the polyploid S phase. Moreover, they indicate that a checkpoint linking the presence of unreplicated DNA t o 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 cycE(01672) cysts frequently differentiate as oocytes, implicating cell-cycle programming in oocyte determinatio n.