THE CELL-CYCLE PROGRAM IN GERM-CELLS OF THE DROSOPHILA EMBRYO

The germ cells of metazoans follow a program of proliferation that is distinct from proliferation programs of somatic cells. Despite their d evelopmental importance, the cell proliferation program in the metazoa n primordial germ cells is not well characterized and the regulatory c ontrols rue not understood. In Drosophila melanogaster, germ cell prec ursors (called pole cells) proliferate early in embryogenesis and then enter a prolonged quiescence. We found that polar nuclear divisions a re asynchronous and lag behind somatic nuclear divisions during syncyt ial cycles 9 and 10. Thus, the polar division program deviates from th e somatic division program when pole nuclei and somatic nuclei still s hare a common cytoplasm, earlier than preciously thought. The lag in p olar nuclear divisions is independent of grapes, which is required for lengthening somatic cell cycles 10-13. Mapping of the last S phase in pole cells and measurement of their DNA content indicate that pole ce lls become quiescent in G2 phase of the cell cycle. We were able to dr ive quiescent pole cells into mitosis by induction of either an activa tor of Cdc2 (Cdc25(string) phosphatase) or a mutant form of Cdc2 that cannot be inhibited by phosphorylation. In contrast, induction of wild -type Cdc2 with a mitotic cyclin did not induce mitosis in pole cells. We propose that inhibition of Cdc2 by phosphorylation contributes to G2 arrest in pole cells during embryogenesis. Furthermore, pole cells enter G1 following induced mitoses, indicating that entry into both mi tosis and S phase is blocked in quiescent pole cells. These studies re present the first molecular characterization of proliferation in embry onic germ cells Of Drosophila. (C) 1998 Academic Press.