Novel Morphogenesis in Ax-3, a Mutant Strain of the Cellular Slime Mould Dictyostelium discoideum

The cellular slime mould Dictyostelium discoideum constructs a succession of characteristic structures (grexes) in the multicellular phase of its life cycle. Compared with the wild-type strain, NC-4, grexes of a mutant strain, Ax-3, exhibited a profound prolongation of the early mound stage of development and a premature and exaggerated tendency to construct short, broad and bulbous forms. The aberrant Ax-3 phenotype was partially corrected by increasing the temperature of development, decreasing the plating density, perfusing with fresh air, or by introducing 0.1 mM-formycin B or 3-deazaadenosine into the agar. Mixtures of NC-4 and Ax-3 formed hybrid grexes which often became transformed into toruses by spiral centrifugal movement of the cells. Under other conditions, hybrid mounds developed into binary grex structures, cylindrical columns on top of hemispherical mounds, each component of which completed the morphogenetic sequence independently. The narrow upper structure appeared to be composed primarily of NC-4 cells. These novel patterns of morphogenesis support our hypothesis that the shape of the organism is controlled by the specification of the circumference of loops of cells within the grex.