embryos show a failure of epidermal cells to undergo mitosis 16.
mutant embryos, division of the NB6-4t lineage is arrested afer the first or second division. However, the first division of this lineage produces one glial cell and one neuronal cell as in wild-type.
Only one, two or three cells are found in the position of the NB7-3 clusters (8%, 75% and 17% respectively) compared to the four seen in wild-type.
In mutant embryos, two to four progeny cells of NB6-4T appear in each hemisegment, as opposed to the 5 to 6 cells seen in wild-type. In the abdominal segment of this mutant, more than half of NB6-4A divided once into 2 cells. The first cell division. the onset of glial differentiation and cell fate bifurcation occur normally in these embryos.
Mitosis is arrested during the 16th cell cycle. Cells are stillable to differentiate and dorsal medial cells carrying bilateral cytoplasmic extensions are clearly identifiable in late embryos. Scer\GAL4sim.PS
-mediated expression of spis.Scer\UAS
increases the number of dorsal medial cells, the implantation of one or two midline cells from these donors into CycA
mutant embryos fails to induce additional dorsal medial cells.
In embryos double mutant for CycA3
, no mitotic figures are observed at mitosis 15. BrdU incorporations reveal that the block is in the preceding G2
Entry into mitosis 16 does not occur in the mutant cells of the epidermis. Cells do not exhibit complete cell cycle arrest.