CELLULAR DIFFERENTL^TION IN THE SLIME MOLD 237 



the agar and subsequently turns the spore masses brown. The genotype 

 can be designated Y+ br~. Mutant wh-1 (white) also aggregates and 

 fruits normally but fails to produce the wild-type (yellow) pigment. 

 Its genotype is therefore Y~ br+. An anomalous I-cell clone was iso- 

 lated from each mutant, and these were grown in mixed culture. A 

 random sample was plated, and one clone of about 2,600 was wild type. 

 This was presumed to be a heterozygote (Y+Y~ br+br~) in which 

 presence of the yellow pigment and absence of the brown were domi- 

 nant. Replating this stock yielded an overwhelming majority of wild- 

 type clones but also a low frequency of segregants (about 0.1 per cent). 

 These were of three phenotypes: the parental brown (Y+ br~); the 

 parental white (Y~ br+); and a recombinant type which produced 

 white spore masses at first and then the red-brown pigment afterward 

 (Y~ br~ ). The fourth segregant would be Y+ br+ {i.e., wild type) and 

 hence indistinguishable from the heterozygote. Upon subculture, the 

 segregant clones bred true to type, but the heterozygote continued to 

 throw segregants at low frequency. The plating of either of the paren- 

 tal stocks and of stable haploid or diploid wild-type strains has never 

 produced comparable variation. 



Chromosome counts and the size distributions of spores and myx- 

 amoebae support the idea that the foregoing represents genetic re- 

 combination. Figure 7 summarizes the spore and myxamoeboid size 

 distributions obtained from the parental stocks, the heterozygote, and 

 segregant isolates. By comparison with the original mutants, the anom- 

 alous I-cell clones derived from them are seen to contain an appreciable 

 contingent of cells in the diploid size range, and this fact is confirmed 

 by chromosome counts of stained preparations. The presumed hetero- 

 zygote distribution is well over toward the diploid range, as it obviously 

 would have to be to maintain a largely wild phenotype by hiding the 

 recessive markers, and this too is confirmed by chromosome counts. 

 Two of the segregant clones, in contrast, appear to be stable haploid 

 stocks like the original mutants, and the third apparently remained 

 anomalous. 



To recapitulate the experimental findings: Wild-type clones are 

 undetectable in pure cultures of the original brown and white mutant 

 stocks or in mixed cultures of the two; wild-type revertants have not 

 been observed in pure cultures of the anomalous I-cell clones derived 

 from these mutants; however, clones bearing the wild phenotype are 

 derivable from mixed cultures of the anomalous clones; such wild-type 

 stocks are not stable but throw segregant clones at low frequency, and 

 these bear either of the parental phenotypes or a recombinant pheno- 

 type. Taken together with the chromosome counts and cell-size dis- 

 tributions, the data seem to us to suggest compellingly that a sexual or 

 parasexual system is operative here. Obviously, more genetic markers 



