Requirement for cell differentiation in Dictyostelium discoideum

Abstract

THE generation of cell diversity during the development of multicellular organisms involves a series of decisions between alternative pathways of differentiation^1,2. In many instances the fate of a cell depends on its position, so that pattern formation and cell differentiation can become facets of the same process³. These phenomena occur in a simple form in the slime mould Dictyostelium discoideum where there are only two major pathways of differentiation—one leading to stalk cells and the other to spores. On starvation the individual amoebae aggregate chemo-tactically to form multicellular masses. Somewhat later the aggregates can be shown to consist of two distinct cell types: prestalk cells which are localised in the anterior and prespore cells in the posterior of the migrating slug⁴. We are interested in how the formation of these cell types is controlled. One approach to this problem involves experimentally inducing differentiation of cells that are somehow prevented from completing normal development. Starving cells plated on agar with a Cellophane overlay aggregate but remain constrained as a monolayer and do not progress to stalk or spore. But addition of 1–5 mM cyclic AMP to the agar results in efficient induction of stalk cell formation provided that a high cell density is used^5–7. At low cell density stalk cells do not form unless a low-molecular weight factor, released by cells at high density, is also provided⁷. We show here that although no mature spores form in conditions of high cell density giving a high yield of stalk cells large numbers of prespore cells do appear. Many of these later transform into stalk cells. Prespore cells do not arise at low density but the density requirement for prespore cell formation, unlike that of stalk cells, cannot be efficiently replaced by the low-molecular weight factor. Five mutants isolated by virtue of their ability to form mature spores under Cellophane likewise do not form spores at low cell density even in the presence of the ‘helping’ factor. Thus entry of developing Dictyostelium cells into the prespore pathway apparently requires some cell interaction different from or additional to that required for entry into the stalk pathway.