FREQUENCY OF SPONTANEOUS BACTERIOPHAGE MUTANTS 



.3. Mottled plaques were often seen on our plates. They generally derive from 

 mutations occurring during plaque formation. In several cases, mottled plaques 

 were present on the same plate with r mutants. Three such mottled plaques were 

 replated and the r component strain was isolated and crossed with a strain from a 

 pure r plaque on the same plate. In two cases wild-type recombinants appeared, 

 indicating that the mottled plaque probably stemmed from an independent r 

 mutation that occurred on the plate; in the third case no recombinant was found. 

 The latter type of mottled plaque could appear in the yield of a bacterium 

 containing a clone of r mutants, owing to some limitations to the complete 

 segregation of virus particles or of their genetic components (Hershey and Chase, 

 1951). 



Discussion 



The exponential rate of gene reproduction in phage, suggested by our analysis 

 of the clonal distribution of spontaneous phage mutants, simply means that the 

 initial gene copy brought in by the infective virus does not possess the monopoly 

 of replication. Its copies act in turn as sources for new replications and the 

 elementary replication process is a reduplication. This conclusion, if correct, 

 represents a step further in our analysis of phage reproduction. 



Our results do not contribute any information as to whether the mutations 

 occur only at reduplication or between reduplications, in the "interphase." They 

 are compatible both with production of phage particles as such and of individual 

 genetic components, although the apparent lack of "sampling losses" is more 

 easily reconciled with the former. 



There is some interest in comparing the clonal distribution of spontaneous 

 mutants with the almost random distribution of recombinant phage from 

 mixed-infected bacteria (Hershey and Rotman, 1949). The very first active 

 phage particles that appear inside mixed-infected bacteria include recombinants 

 (Doermann and Dissosway, 1949), whose distribution is also intermediate 

 between a clonal, reduplicational one and a random one. The facts suggest the 

 following conclusions: 



1. Recombination occurs late in reproduction of the genetic material of phage. 

 This may be due either to the coincidence of recombination with some terminal 

 step in phage maturation or to an increased probability of recombination when 

 large numbers of phage elements are present in a cell. 



2. Recombinants detectable as mature particles around the middle of the 

 intracellular growth period do not reduplicate as such; otherwise, by giving rise 

 to clones, they would cause the later population of recombinants to be distributed 

 more and more like spontaneous mutants, contrary to experimental finding 

 (terminal distribution of recombinants almost random). 



3. Reproduction of the genetic material of phage, therefore, takes place 

 mainly by reduplication of elements that are not yet in the form of mature phage 

 particles. 



149 



