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

¹ Dept. of Biological Sciences, Northwestern University, Evanston, Illinois

1. A study has been made of the formation of aggregative centers by mixed populations of D. 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:

(a) Under conditions where the wild type is too dispersed to aggregate alone, addition of aggregateless cells permits the formation of aggregative centers. When the number of centers so formed is plotted against the number of variant cells present, a saturation curve is obtained.

(b) For the case where the aggregateless cells are present in excess, the number of centers formed is directly proportional to the number of wild type added.

(c) Neither the total population nor the proportion of wild type cells changes significantly during the period of incubation prior to the onset of aggregation.

2. These 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 individuals and the aggregateless variants are the "responding cells" which can aggregate only in the presence of an initiator.

3. Estimates made of the proportion of initiators in the wild type population, using as test systems the wild type population itself as well as two aggregateless variants, range between one per 980 and 2110 individuals. The discrepancy is considered to be real and an explanation is offered.