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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 Stringcdc25;
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.
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