Symmetry breaking in Dictyostelium morphogenesis:: Evidence that a combination of cell cycle stage and positional information dictates cell fate

The event that visibly breaks the symmetry of the Dictyostelium aggregate is the formation of a nipple-shaped tip at its apex, but there has been considerable debate as to the equivalence of cells entering development, particularly with regard to the effect of cell cycle position at the onset of starvation. We show that there is a strong correlation between cell cycle position at the time of starvation and the subsequent expression of a marker of tip cell differentiation. Cells starved in late-G2 phase selectively differentiate into tip cells. Previous evidence indicated that tip cells differentiate at the extreme periphery of the aggregate and then move to the apex. Taken in combination, these data suggest that cell cycle position at the onset of starvation affects differentiation fate by determining a cell's probable position within the aggregate, i.e., that both cell cycle and positional information are used to generate pattern. (C) 1997 Academic Press.