Fig. 2. Restoration of terminal mitosis 16 in the epidermis of CycA mutant embryos requires the simultaneous expression of stabilized B-type cyclins and String/Cdc25-independent Cdk1. (A-X) Heat-inducible transgenes allowing expression of the String/Cdc25 phosphatase-independent Cdk1^AF mutant (K1^AF), wild-type Cdk1 (K1), String/Cdc25 (Stg), Cyclin B (B), Cyclin B3 (B3), or stabilized versions of these B-type cyclins lacking the degradation signals for APC/C-mediated proteolysis (B^s and B3^s) were crossed into the CycA^C8LR1 mutant background. Embryos before mitosis 16 were either exposed to a heat shock (+hs) or not (-hs). After ageing to stage 13, embryos were labeled with anti-tubulin (Tub), a DNA stain (DNA) and anti-ß-galactosidase (not shown) for the identification of CycA^C8LR1 mutant embryos (CycA^-) and sibling control embryos (CycA⁺). E-L present high magnification views from the embryos shown in A-D. Cell density in CycA mutants after failure of the terminal mitosis 16 is only half of that observed in control embryos (compare A,E,F with B,G,H) and is not increased by Cdk1^AF expression (compare B,G,H with D,K,L). Co-expression of Cdk1^AF and stabilized B-type cyclins, however, restores entry into the terminal mitosis 16 in CycA mutants and leads to an arrest in this mitosis in both control (M,N) and CycA mutants (O,P), as evidenced by the presence of spindles and condensed chromosomes. Co-expression of wild-type Cdk1 and stabilized B-type cyclins leads to an arrest during mitosis 16 in control embryos (Q,R), but does not restore mitosis 16 in CycA mutants (S,T). Cdk1 activators carrying degradation signals for APC/C-mediated proteolysis also fail to restore mitosis 16 in CycA mutants. Neither expression of a combination of Cdk1^AF with wild-type B-type cyclins (U,V) nor expression of wild-type String/Cdc25 phosphatase with stabilized B-type cyclins (W,X) restores the epidermal cell density in the CycA mutants to normal. The numbers in lower right panel corners represent the cell counts in the regions of the right segments shown in E-X.