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.