Strains of Dictyostelium discoideum were paired in various combinations. Certain pairings resulted in the production of macrocysts, which are thought to be the sexual stage in the cellular slime molds. One selfcompatible strain was found. Other strains produced macrocysts only when paired with an alternate mating type. Two strains were found to form macrocysts when paired with either mating type but not alone or with each other. Several strains did not form macrocysts under any circumstances. Strains were graded according to their mating competency based on the number of macrocysts produced relative to the aggregated myxamoebae observed in the pairings.Macrocysts were first described by Blaskovics and Raper (1) as heavy-walled structures of multicellular origin that developed after cell aggregation in some isolates of Dictyostelium mucoroides and D. minutum and represented a morphogenetic phase alternative to sorocarp formation. Whether formed singly or in clusters, they noted that development began at the center of each incipient macrocyst and proceeded outward, the constituent myxamoebae seemingly being transformed into cells with semirigid walls, termed endocytes. They speculated that the myxamoebae became walled since they had strongly refractile boundaries. Later, these endocytes disappeared and the contents of the cyst became homogeneous.Subsequent ultrastructural studies have shown that a large cell, termed a cytophagic cell (2) or a giant cell (3), arises in the population and progressively engulfs the surrounding myxamoebae, which, once enclosed in vacuoles, represent the endocytes. Before this, the associated myxamoebae form a loose, fibrillar wall (primary wall) around the nascent cyst similar to the slime sheath that surrounds the pseudoplasmodium and the developing sorocarp. After engulfment of the myxamoebae is complete, the giant cell secretes a rigid, cellulosic wall (secondary wall) and later a thicker, more flexible, trilaminar wall. Coincident with the formation of this tertiary wall the endocytes are reduced to small fragments and ultimately disappear, giving the cyst a homogeneous appearance. After a variable period of dormancy, depending upon the species, the single large protoplast of the macrocyst undergoes progressive cleavage to give rise to trophic amoebae that emerge upon germination (4).Factors affecting the formation and germination of macrocysts have been studied (5, 6). In general, macrocyst formation is favored by darkness, very wet cultural conditions, temperature at the higher range of tolerance of the organism, and the absence of phosphate in the medium. The opposite conditions favor sorocarp production. Macrocysts that have undergone the appropriate period of dormancy can be induced to germinate by placing them in the light at low temperature (15-200).There is strong evidence to indicate that the macrocyst represents the true sexual stage in the cellular slime molds. In Polysphondylium violaceum (3), the giant cell has been shown to be at first binucleate but to be un...
