DROSOPHILA PLUTONIUM PROTEIN IS A SPECIALIZED CELL-CYCLE REGULATOR REQUIRED AT THE ONSET OF EMBRYOGENESIS

Unfertilized eggs and fertilized embryos from Drosophila mothers mutan t for the plutonium (plu) gene contain giant polyploid nuclei resultin g from unregulated S-phase. The FLU protein, a 19-kDa ankyrin repeat p rotein, is present in oocytes and early embryos but is not detectable after the completion of the initial rapid S-M cycles of the embryo. Th e persistence of the protein during the early embryonic divisions is c onsistent with a direct role in linking S-phase and M-phase. When ecto pically expressed in the eye disc, FLU did not perturb the cell cycle, suggesting that FLU regulates S-phase only in early embryonic develop ment. The pan gu (png) and giant nuclei (gnu) genes also affect the S- phase in the unfertilized egg and early embryo. We show that functiona l png is needed for the presence of FLU protein. By analyzing png muta tions of differing severity, we find that the extent of the png mutant phenotype inversely reflects the level of FLU protein. Our data sugge st that FLU protein is required at the time of egg activation and the completion of meiosis.