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SUMMARY : Study of three mutants of Dictyostelium discoideum-Fruity, Bushy and Dwarf-provide specific examples of aberrant behaviour during aggregation and the construction of fruiting bodies. The proportion of initiator cells in the population and the optimal population density for aggregation have been shown to be under genetic control. In the Fruity mutant, the great increase in the number of initiator cells leads to the formation of aggregates composed of very few individuals and cor- respondingly tiny fruits. The morphology of the aggregates and fruits and the regulatory properties of this morphogenetic system have been examined. Slime mould myxamoebae, otherwise typical Protista, collectively display a remarkable developmental capacity leading to the formation of organized multicellular structures (Raper, 1941 ; Bonner, 1944; Sussman, 1955). Aggre- gation, the part of the morphogenetic sequence which is of chief concern here, serves as a device to bring the hitherto independent cells into a compact mass. It begins after the stationary growth phase has been reached, the myxamoebae lengthening greatly and orienting radially. The cells then move in toward centres of aggregation, at first as individuals and later in ramified cell streams. The conical aggregate which results is transformed into an organized pseudo- plasmodium which migrates over the agar surface and ultimately gives rise to a sessile fruiting body consisting of a spore mass, a parenchymatous stalk, and a basal disk. Some insight into the mechanisms underlying the aggregative process has been gained as a result of previous investigations. The response of the outlying cells to the aggregative stimulus has been shown by a variety of means to be chemotactic (Runyon, 1942; Bonner, 1947, 1949; Shaffer, 1953; Sussman & Lee, 1954, 1955). The attracting substances, given the generic name ' acrasin' by Bonner, are produced at the aggregative centres and along the streams and are readily diffusible and highly unstable. That both the agents and the chemo- tactic responses are specific has been indicated in studies of mixed aggregations by cells of different species and genera (Raper & Thorn, 1941) and by treatment of myxamoebae with washings obtained from heterologous aggregates and pseudoplasmodia (Shaffer, 1953). The aggregation of myxamoebae on a washed agar distilled water substratum has been examined under conditions where the cells remained constant in number and where rigorous control over population size and density could be achieved (Sussman & Noel 1952; Sussman, 1952). The population density (no. cells/mm.2) radically influenced the number of

'Fruity' and other Mutants of the Cellular Slime Mould, Dictyostelium discoideum: a Study of Developmental Aberrations