THE INFLUENCE OF LIGHT ON THE SIZE OF AGGREGATIONS IN DICTYOSTELIUM DISCOIDEUM

¹ Department of Bacteriology, University of Wiscosin, Madison, Wisconsin 53706
² Department of Botany, University of Wiscosin, Madison, Wisconsin 53706

1. Myxamoebae of Dictyostelium discoideum, NC-4(H), were pregrown in the dark on Escherichia coli or Aerobacter aerogenes, washed and deposited on non-nutrient agar. Populations incubated in constant light produced more and smaller aggregations and sorocarps than similar populations incubated in the dark. If populations were incubated in darkness for several hours and then transferred to light, the number of aggregations was reduced and the dimensions of these and the resulting sorocarps were correspondingly greater. The rate of increase in the number of aggregations was most rapid if the myxamoebae were exposed to a long initial dark period followed by light.

2. The chemotactic response of myxamoebae incubated in light or darkness was studied by depositing "attracting" and "responding" cells in separate small populations at predetermined distances from each other. The sphere of attraction by myxamoebae aggregating in light was found to extend over a shorter distance than that of cells aggregating in darkness. Among other possibilities, inactivation of the attracting substance (s) in the light may account for reduced attraction, hence result in smaller aggregations.