Absence of Cell Sorting Out in the Grex of the Slime Mould Dictyostelium discoideum

Abstract

THE development of the cellular slime moulds demonstrates very simple pattern formation. Cells from an apparently homogeneous population come together to form an aggregate which differentiates into a pattern of only two types of cell, stalk and spore. These are rearranged spatially to form the fruiting body. A striking feature of this pattern is that the proportions of stalk and spore are relatively constant over a wide range of size of aggregate¹. Although the aggregate can differentiate directly into a fruiting body² it usually migrates as a coordinated irritable whole in the form of a tapered cylinder called the grex. It is known that any sufficiently large part of the migrating grex can give rise to a normal fruiting body, and, in this sense the development of the slime mould shows considerable regulation^3,4. At present most workers accept that during migration the front third of the grex becomes prestalk cell and rear two-thirds prespore¹. It is necessary for an understanding of pattern formation to know when this pattern is specified and when the cells become determined. In view of the capacity of the grex to regulate, it is of great interest that histochemical and ultrastructural differences between prespore and prestalk cells in the grex have been found by various workers^1,5–9, because such differences are clearly reversible.