Jl. Clay et al., INITIAL CELL-TYPE CHOICE IN A SIMPLE EUKARYOTE - CELL-AUTONOMOUS OR MORPHOGEN-GRADIENT DEPENDENT, Developmental biology, 172(2), 1995, pp. 665-674
Dictyostelium discoideum is a simple eukaryote that lives as an amoeba
until starvation triggers aggregation. The aggregate forms a slug whi
ch then develops into a fruiting body with two main cell types, stalk
and spore cells. Two mechanisms have been proposed to explain cell-typ
e differentiation. Studies using expression of the ecmA gene as a pres
talk cell marker indicated that gradients of morphogens determine cell
fate in the slug. However, studies using dyes or the cysteine protein
ase 2 (CP2) gene product as a prestalk cell marker indicated that cell
autonomous factors such as cell-cycle phase at the time of starvation
cause an initial choice of cell fate. To help resolve these differenc
es, we have used transformed cells containing the promoter of the pres
talk gene ecmA fused to beta-galactosidase (Jermyn and Williams, 1991)
to study the differentiation of Dictyostelium cells at low cell densi
ty, at which cell-to-cell interactions and morphogen gradients are min
imal. We find that under all conditions of low cell density in which e
xpression of the ecmA fusion gene occurs, it is invariably detected in
less than 25% of the cells from a clonal population. This suggests th
at a cell-autonomous mechanism is involved in ecmA expression. We then
used double-labeled immunofluorescence to examine the ontogeny of the
CP2-positive and the ecmA-positive cells. In developing aggregates, 5
, to 12% of the cells are CP2-positive from 12 to 24 hr of development
. The ecmA-positive cells are first detected at 16 hr as a subset of t
he CP2-positive cells and then increase in number. At approximately 20
hr, the CP2-positive cells and the ecmA-positive cells are almost com
pletely overlapping sets. By late development, all of the CP2-positive
cells are ecmA-positive and an additional 10% of the CP2-negative cel
ls are also ecmA-positive. This indicates that up to 20 hr of developm
ent, ecmA is expressed only in CPP-positive cells. The data thus sugge
st that cell-cycle phase at the time of starvation causes an initial c
hoice of cell type and that during later development other factors inf
luence cell fate. (C) 1995 Academic Press, Inc.