Drosophila embryonic pattern repair: how embryos respond to cyclin E-induced ectopic division.

The Drosophila melanogaster embryo ordinarily undergoes thirteen cycles of rapid syncytial division followed by three rounds of cellular division for most cells. Strict regulation of the number of divisions is believed to be essential for normal patterning and development. To determine how the embryo responds to hyperplastic growth, we have examined epidermal development in embryos that experience additional rounds of mitosis as the result of ectopic Cyclin E expression. We observed that the cell density in the epidermis nearly doubled within 1 hour of Cyclin E induction. The spacing and width of the ENGRAILED and wingless stripes was unchanged, but the cell density within the stripes was increased. By 4 hours after Cyclin E induction, the cell density had returned to almost normal values. The embryos developed, albeit more slowly, to produce viable larvae and adults. The excess cells were removed by apoptosis in a reaper-dependent fashion as evidenced by increased reaper expression. Embryos lacking cell death in the abdomen exhibited changes in ENGRAILED expression. In addition, germband retraction and dorsal closure were slower than normal. Ectopic Cyclin E expression in cell-death-deficient embryos exacerbated the germband retraction and ENGRAILED-expression defects.