Aberrant pattern formation in myosin heavy chain mutants of Dictyostelium

In mutant Dictyostelium strains that fail to accumulate the myosin heavy chain (MHC A), development is relatively normal up to the tight aggregate stage but is arrested prior to formation of the apical tip (DeLozanne and Spudich 1987, Knecht and Loomis, 1987). We show that in aggregates formed by such MHC A deficient (MHC A-) strains the proportions of pstA and pstB cells, the two prestalk cell types, and of prespore cells are similar to those found during normal development but their distribution is radically different. During the initial stages of normal slug formation, pstA cells move to the tip, pstB cells accumulate in the base and prespore cells occupy the remainder of the aggregate. In the aggregates initially formed by MHC A- mutants pstA cells are present in a central core, pstB cells are present in the cortex and prespore cells lie sandwiched between them. Eventually, cells within the cortex differentiate into mature stalk cells but spores are never formed. Mixing experiments, in which MHC A- cells are allowed to co-aggregate with an excess of normal cells, show that MHC A- prestalk cells enter the aggregate relatively normally but are unable to enter the slug tip or to migrate into the stalk at culmination and that MHC A- prespore cells accumulate in the lower part of the spore head during culmination. Thus MHC A- cells appear to be able to move within the multicellular aggregate but are incapable of participating in normal morphogenesis. The structures formed by MHC A- cells are very similar to those of the agglomerates that form when wild-type cells are developed in roller-tube culture, conditions that result in loss of the polarity imparted by the presence of an air-water interface. We propose formation of such a structure by MHC A- cells to be a default response, caused by their inability to undertake the shape changes and intercalatory cell movements that are necessary to form and extend the tip.

Cellular differentiation and pattern formation in the absence of morphogenesis in the cellular slime mould Polysphondylium pallidum: evidence for a biochemical tip (organizer) in submerged aggregates

When amoebae of Polysphondylium pallidum WS320 are placed in nonnutrient buffer in roller tube culture they form spherical or ellipsoidal aggregates. At first the aggregates demonstrate a "loose" morphology but by 12 h, with the formation of a cellulose-containing, peripheral sheath, they become "tight" aggregates. At this time stalk differentiation begins. Using various methods for the resolution of prespore (ultrastructure, spore antigen immunofluorescence, periodic acid – Schiff staining) and prestalk (ultrastructure, alkaline phosphatase histochemistry, neutral red staining, Calcofluor fluorescence) cell localization, the pattern of cell differentiation in submerged aggregates was shown to be essentially identical to that of normal pseudoplasmodia. Furthermore, using a cAMP bioassay it was revealed that the submerged aggregates, while devoid of a morphological tip, do possess a biochemical tip which is correlated with sites of neutral red staining and stalk cell differentiation. As a result of these studies, an earlier argument that the tip of the pseudoplasmodium is not essential for the establishment of pattern or in the "organization" of cellular differentiation during slime mould development is contradicted.