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CELLLLAR DIFFERENTIATION 

 IN THE SLIME MOLD* 



Maurice Sussman 



BRANDEIS UNIVERSITY 



In one respect, at least, the cellular slime molds are schizoidal organ- 

 isms. ( Some would gaily extend the classification to cover their investi- 

 gators as well; I shall not.) As long as slime-mold myxamoebae con- 

 tinue to grow, they remain typical Protista, independent of one another 

 to the same extent as the members of any microbial population. Each 

 cell can act individually as the functional unit of existence, of reproduc- 

 tion, genetic transmission, selection, and, indeed, of evolution. But once 

 having ceased to grow, the cells collect in multicellular aggregates and 

 assume a metaphytic or metazoan (depending on one's bias) condi- 

 tion. They submerge their individuality in an organized multicellular 

 structure which becomes the new functional unit of existence. 



In Dictyostelium discoideum, each aggregate is transformed into a 

 slug-shaped pseudoplasmodium which migrates over the substratum 

 and ultimately constructs a fruiting body composed of a spore mass and 

 a stalk and basal disc. The cells inhabiting these regions are morpho- 

 logically and functionally distinguishable and display a degree of phen- 

 otypic heterogeneity qualitatively commensurate with what one would 

 ordinarily call cellular differentiation in a higher plant or animal 

 (Raper, 1941; Raper and Fennell, 1952). Phenotypic heterogeneity is 

 also apparent much earlier in the morphogenetic sequence, as the cells 

 proceed through the stages of aggregation and pseudoplasmodial mi- 

 gration (Bonner, 1949, 1952, 1957; Bonner et al., 1955; Krivanek and 

 Krivanek, 1956, 1958; Raper, 1940, 1941). These events are accom- 

 panied by the appearance of antigenically active macromolecules 



* The work reported in this paper was supported by grants from the National 

 Cancer Institute and the National Science Foundation. 



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