Fig. 1. Cyclin A is preferentially required for terminal mitoses. (A-J) Mitotic cells were labeled with antibodies against phosphohistone H3 (A-D,G-J; pH3) and ß-galactosidase for genotyping (not shown). Mitotic cells are observed throughout the thoracic and abdominal epidermis in sibling control embryos at the stage of mitosis 16 (A,B; CycA⁺). By contrast, mitotic cells are restricted mainly to the prospective anterior spiracle region in the CycA^C8LR1 mutant epidermis (C,D; CycA^-; white arrow). Cells in this restricted epidermal region are exceptional because they progress through an additional division cycle 17 after mitosis 16 during wild-type embryogenesis, as indicated by BrdU pulse labeling (BrdU) of control embryos at the stage of S phase 17 (E,F; white arrow), while essentially all the other epidermal cells exit from the mitotic cycle after mitosis 16. Mitotic divisions were also observed in the central nervous system of stage 14 CycA^C8LR1 embryos (I,J; CycA^-). B,D,F,H,J present high magnification views from the embryos shown in A,C,E,G,I, respectively. Inset in J illustrates the presence of normal anaphase figures in late CycA^C8LR1 embryos, with DNA and anti-phospho-histone H3 staining shown in red and green, respectively. (K) Embryos at the stage of mitosis 16 were immunolabeled with anti-tubulin and anti-ß-galactosidase for genotyping. The number of mitotic cells in thoracic segments 1-3 (see 1, 2 and 3 separated by dashed lines in A) was counted in control (+) and CycA mutants carrying various alleles (C8, CycA^C8; 114, CycA^neo114; 183, CycA¹⁸³; C8LR1, CycA^C8LR1). Columns represent the average number of mitotic cells (n=10 embryos).