Fig. 3. How changes in regulation can modify cell cycle types. (A) In the Drosophila oocyte cyst, there is a transition from somatic cell cycles to endocycles during oocyte maturation in the nurse and follicle cells, whereas the oocyte is maintained in prophase arrest during Meiosis I. As nurse cells and the oocyte are coupled cytoplasmically, an important question is how these cells maintain different cell cycle states. Lilly described (see main text) how this appears to be regulated by the local modulation of Dacapo levels, which regulate CycE/Cdk2 activity. The pathways mediating this mechanism are not yet known. Ruholla-Baker described how the transition to endocycles in the follicle cells is mediated by activation of the Notch pathway through Delta ligand expressed by the oocyte and nurse cells (see main text). Notch signaling may regulate the cell cycle in at least three ways: repression of CycB/Cdk1 activity by downregulation of String^cdc25; upregulation of Fzr/Cdh1-dependent APC activity; and activation of CycE/Cdk2 activity by downregulation of Dacapo. (B) In Arabidopsis and Medicago trunculata, the transition from a somatic cell cycle to an endocycle during differentiation appears to depend on the activity of the CDK CdkB1;1. This may involve Fzr/Cdh1-dependent APC activity and downregulation of CycD3;1, although the specific nature of the interactions of these proteins with CdkB1;1 is not yet known.