232 CELLS, TISSUES, AND ORGANISMS 



Upon bacteria, and even increased in size. They were never seen to 

 revert to the R-cell phenotype except through cell division. 



2. Cell division caused the I-cells to revert at a very high rate. 

 Rarely did they breed true for a generation and yield two I-cell 

 daughters. More commonly they bred "semiclonally" to give one daugh- 

 ter of each phenotype. The remainder reverted in the first generation 

 by producing only R-cell daughters or cells of a size intermediate be- 

 tween the two ( i.e., in the diploid range seen in Figure 4 ) . 



3. Because of this instability, by the time the clones had reached 

 the size of a few hundred individuals the proportion of I-cells declined 

 to zero in practically all or to 1 to 2 per cent in just a few. Subsequent 

 development caused 90 per cent of the clones to become indistinguish- 

 able from those derived from R-cells. That is, I-cells began to reappear 

 as the clonal size exceeded 500 to 1,000 cells, and the ratio of I-cells to 

 R-cells ultimately reached the usual steady-state level of 1:2000. The 

 general cell-size distributions were the same as those shown for NC-4 in 

 Figures 3 and 4. Aceto-orcein-stained squashes revealed the presence 

 of haploid chromosome sets only. The aggregative performances, one 

 example of which is seen in Figure 5, were indistinguishable from 

 those of the R-cell clones or of the carried stock. 



In contrast, 10 per cent of the clones derived from I-cells displayed 

 anomalous properties. The anomalies: 



1. Ploidy: Chromosome counts revealed that these clones were a 

 mixture of haploid and diploid cells. 



2. Size distribution: The presence of diploid cells was reflected in 

 the altered size distributions both of spores and myxamoebae. For ex- 

 ample, histograms of anomalous I-cell clone I-2A, shown in Figures 3 

 and 4, are seen to straddle both the haploid and diploid size ranges. 



3. I-cell frequency: The proportion of amoebae classifiable as I- 

 cells increased markedly. In clone I-2A the ratio is approximately 

 1:100; in others the frequency is somewhat less. 



4. Aggregative performance: The anomalous clones formed many 

 more centers of aggregation than the normal I-cell clones or clones 

 derived from R-cells or the carried NC-4 stock. Moreover, they did so 

 at markedly lower population densities. Figure 5 illustrates this. 



It must be emphasized that these anomalies are inherited in very 

 stable fashion through many subcultures and clonal reisolations; i.e., 

 the anomalous I-cell clones are true variant strains. The ploidal mix- 

 ture can be derived from any cell in the population. That is to say, each 

 cell possesses the genetic potential for yielding a population in which 

 there is a rapid shuttling back and forth between the haplo-phase and 

 the diplo-phase. Thus the anomalous I-cell clones inherit in very stable 

 fashion a penchant for instability! Experiments have indicated that the 



