AN ANALYSIS OF THE AGGREGATION STAGE IN THE DEVELOPMENT OF THE SLIME MOLDS, DICTYOSTELIACEAE. II. AGGREGATIVE CENTER FORMATION BY MIXTURES OF DICTYOSTELIUM DISCOIDEUM WILD TYPE AND AGGREGATELESS VARIANTS

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Abstract
A study was made of the formation of aggregative centers by mixed populations of Dictyostelium discoideum wild type and aggregateless variants. The latter are capable of responding to the aggregative stimulus but not of producing it. The following results were obtained: Under conditions where wild type cells were too dispersed to aggregate alone, addition of aggregateless cells permitted the formation of aggregative centers. The numbers of centers so formed plotted against the number of aggregateless cells added provides a saturation curve. In the presence of excess aggregateless cells, the number of centers formed was directly proportional to the number of wild type cells present. Neither the total population or proportion of wild type cells changes significantly during the incubation period prior to aggregation. The findings demonstrate that the evocation of an aggregative center is accomplished by a single individual in the wild type population. These have been termed "initiator" cells. Under the conditions employed, the remainder of the wild type population consists of "responding" cells which can aggregate only in the presence of an initiator. Estimates made of the proportion of initiator cells ranged between one per 980 and per 2110 individuals, using the wild type cells alone and 2 strains of aggregateless variants as the test systems. The discrepancy is considered to be real and an explanation is offered.