Fig. 3. Cyclin A prevents a premature disappearance of Cyclin B3 before mitosis 16. (A-C) The prd-Gal4 transgene was used to drive UAS-CycA expression in alternating segments of CycA^C8LR1 mutant embryos. Embryos were immunolabeled with anti-Cyclin B3 (CycB3, green in C) and a DNA stain (red in C). The UAS-CycA expressing segments (white arrowheads in A) contain more anti-CycB3-positive cells than do the segments without UAS-CycA expression. Among the segments without UAS-CycA expression, however, the one with most of the prospective anterior spiracle region (white arrow) has an exceptionally high number of CycB3-positive cells. The high magnification views in B and C indicate that mitosis 16 has already been completed in the anti-CycB3-negative regions in the segment with UAS-CycA expression to the right of the dashed white line. These regions (see for example cells around `1+') are characterized by a higher density of cells with smaller nuclei than in regions with CycB3-positive cells in G2 before mitosis 16 (around `2+'). The anti-CycB3-negative region `1-' in the segment without UAS-CycA expression to the left of the dashed line, which corresponds to region `1+', has a low cell density, reflecting the failure of mitosis 16. Importantly, in this segment without UAS-CycA expression, Cyclin B3 is not only absent from region `1-' but also from the region around `2-', indicating that in the absence of Cyclin A, Cyclin B3 disappears prematurely before mitosis 16, as the corresponding region `2+' in the segment with UAS-CycA expression still consists of anti-CycB3-positive cells in G2 before mitosis 16.