In Dictyostelium, initial cell type choice is correlated with the cell
-cycle phase of the cell at the time of starvation. We have isolated a
mutant, ratioA (rtoA), with a defect in this mechanism that results i
n an abnormally high percentage of prestalk cells. The rtoA gene has b
een cloned and sequenced and codes for a novel protein. The cell cycle
is normal in rtoA. In the wild type, prestalk cells differentiate fro
m those cells in S or early G(2) phase at starvation and prespore cell
s from cells in late G(2) or M phase at starvation. In rtoA mutants, b
oth prestalk and prespore cells originate randomly from cells in any p
hase of the cell cycle at starvation.