Cell aggregation in Dictyostelium discoideum

Abstract

The capacity of small populations of myxamebas of Dictyostelium discoideum to aggregate, i.e., to form pseudoplasmodia, was investigated under contrasting cultural and environmental conditions, including: (1) populations developed in situ on washed or purified agar where growth of myxamebas was restricted by limiting the nutrient (bacteria), and (2) populations of pregrown myxamebas that were harvested, washed, and deposited in known numbers at predetermined densities upon washed or purified agar varying in composition and prepared in different ways. The investigations centered upon the type culture of Dictyostelium discoideum (strain NC-4) and certain substrains derived from it, but included also nine other isolations of this species unrelated to NC-4 in point of origin. Aggregative capacity was found to be similar in all these when small populations were grown and tested under like conditions. The conditions under which tests of aggregative capacity were made profoundly influenced the results obtained. Factors of primary importance to aggregation seemed to include the following: (1) the existence of a critical relationship between the number of myxamebas and their density per unit area of substrate; (2) the necessity for cells to remain within reasonable proximity over a period of several hours subsequent to nutrient depletion in undisturbed cultures or after deposition on agar of pregrown populations in minute drops of water or saline; (3) the removal by washing from normal agar of certain unidentified substances that permit spreading of implanted drops and accentuate cell dispersal; and (4) the optimal reconstitution and/or preparation of the agar gel. The ability of a small population of 100–150 myxamebas, or fewer, to aggregate was found to be independent of any special type of cell recognizable by the writers. Rather this capacity is thought to result from physiological changes emergent within the population as a whole following nutrient depletion, on the one hand, or implantation of a pregrown population on the other. Unusually large myxamebas, believed to represent the so-called “I-cells” reported in existing literature, were occasionally observed; but such did not occur in any reasonably definite ratio in our cultures, and their diverse characteristics seemed to suggest no common origin. Finally, their total absence from the great majority of our test populations fails to support the notion that they represent a unique cell type of special significance to pseudoplasmodium formation in Dictyostelium discoideum.