Rh. Gomer et Rr. Ammann, A CELL-CYCLE PHASE-ASSOCIATED CELL-TYPE CHOICE MECHANISM MONITORS THECELL-CYCLE RATHER THAN USING AN INDEPENDENT TIMER, Developmental biology, 174(1), 1996, pp. 82-91
Upon starvation, cells of the simple eukaryote Dictyostelium discoideu
m aggregate and differentiate into several cell types. Two main cell t
ypes are prestalk and prespore, which later usually become stalk and s
pore cells. The differentiation is plastic, and several factors can al
ter cell-type ratios. Two mechanisms have been proposed to regulate th
e initial differentiation. One hypothesis is that different levels of
extracellular factors within the aggregate determine initial cell type
. We and others have proposed that cell type is initially determined b
y cell-cycle phase at the time of starvation: prestalk cells are deriv
ed from cells which, at the time of starvation, happen to be in a roug
hly 2-hr-long sector of the cell cycle which overlaps S and early G2 a
nd that certain extracellular factors are then used to maintain the pr
oper prestalk: prespore ratio and to control later stages of developme
nt such as the prestalk-to-stalk conversion. To examine the relationsh
ip between initial cell-type choice and the cell cycle, and how this 2
-hr-long sector is generated, we increased the length of S phase by mi
ld treatments of cells with DNA-synthesis inhibitors. When the fractio
n of the cell cycle occupied by S phase is increased and the cells are
then starved, the prestalk:prespore ratio increases. This increase wa
s observed using two markers for prestalk cells, CP2 and ecmA::lacZ. I
n addition, there is a close correlation between the fraction of the c
ell cycle occupied by S phase and the prestalk:prespore ratio, irrespe
ctive of total cell-cycle length. These results validate the hypothesi
s that the initial choice of cell type is determined by cell-cycle pha
se at the time of starvation, and indicate that the cell-type choice m
echanism monitors the cell cycle rather than using an independent 2-hr
-long timer started at the beginning of S phase. (C) 1996 Academic Pre
ss, Inc.