A study of PstB cells during Dictyostelium migration and culmination reveals a unidirectional cell type conversion process

Abstract

The prestalk region of the Dictyostelium slug has recently been shown by Williams and his collaborators to consist of two distinct cell types, pstA and pstB cells. Here the movement of these cells in both the slug and culmination stages has been examined with the use of vital dyes. In the slug some of the pstB cells are continually lost from the prestalk region as small clusters of cells. These cells move through the prespore region and temporarily lie in the rearguard region at the posterior end of the slug. They are finally left in the slug's slime track as single cells or groups of a few cells. When culmination is initiated the pstB cells move as a whole from the prestalk region to the base where they join the rearguard cells to form the basal disc of the fruiting body. Transplantation experiments reveal that the rearguard cells form an outer ring portion of the basal disc and the pstB cells form an inner portion to which the stalk attaches. The continuous loss of one cell type during the slug stage without any change in cell type proportions suggests that cell types are redifferentiating. Grafting and transplantation experiments reveal that there is a unidirectional flow of cells through successive steps of cell type conversion. Prespore cells redifferentiate as anterior-like cells which migrate to the prestalk region and become pstA cells. The pstA cells then replace the pstB cells that are lost from the slug.

Patterns of cell movement within the Dictyostelium slug revealed by cell type-specific, surface labeling of living cells

“…Such an accumulation of ALCs is often seen in normal slugs, and the structure so formed is called the rearguard. It may result from an abortive attempt at culmination because, during culmination, a subset of the ALCs move basipetally to form the outer part of the basal disc (Sternfeld and David, 1982;Jermyn and Williams, 1991;Sternfeld, 1992).…”

“…Prestalk cells, which comprise about 13% of cells within the slug (Sternfeld and David, 1962), are stained selectively by vital dyes such as neutral red (Bonner, 1962). Interspersed among the prespore cells in the posterior region are cells that stain with vital dyes and that are therefore called ants rior-like cells (ALCs) (Sternfeld andDavid, 1961, 1962).…”

“…During culmination, the ALCs move to surround the spore head to form cups that cradle it, and they also form the outer part of the basal disc, the structure that supports the stalk (Sternfeld and David, 1962;Jermyn and Williams, 1991;Sternfeld, 1992). In contrast, the prestalk cells move into the stalk tube where they vacuolate and die (Raper and Fennell, 1952).…”

Two distinct populations of prestalk cells within the tip of the migratory Dictyostelium slug with differing fates at culmination

“…At culmination the ALCs move to surround the emerging spore head to form the upper and lower cups, structures that cradle the spore head (Sternfeld and David, 1982;Jermyn and Williams, 1991). They also form the outer part of the basal disc, a conical-shaped structure that supports the culminant Sternfeld, 1992). In the migratory slug, the ecmA and ecmB genes are expressed at a low level in a subset of the ALCs, with some ALCs expressing both genes, some expressing one or the other gene and some ALCs apparently expressing neither gene (Gaskell et al, 1992).…”

“…They were originally designated pstB cells but, because they have subsequently been shown to express both the ecmA and ecmB genes, they have been renamed pstAB cells (Gaskell et al, 1992). The core of pstAB cells seems to constitute a type of stalk tube primordium that may result from periodic, abortive attempts by the slug to enter culmination (Sternfeld, 1992).…”