  In Dictyostelium discoideum the proportion of cell types is known to be
actively regulated, recovering, for example, after the removal of a given cell
type. However, we have recently shown that regulation is intrinsically
imprecise: it controls the proportion above/below certain upper and lower
thresholds, but not within an allowed range of values that these thresholds
define. To explain this finding we present a model based on (i) a global
negative feedback, and (ii) a cell-autonomous positive feedback leading to a
hysteresis-like behaviour (i.e, a bistability of cell type). The simulated
cell-type proportion is indeed found to span a range of values as a consequence
of the bistability. We conclude that there is a general conflict between the
precision of proportion regulation and the robustness of the differentiation of
cell types.
