GENETIC RECOMBINATION IN BACTERIAL VIRUSES 3 



In 1945 another niurant was found by Hershcy at Washington 

 University (Hershcy, 1946). This mutant was called r. It gave 

 much larger plaques than the wild type r+, and it was directly recog- 

 nizable on the plate. Skeptical of mutual exclusion between two parti- 

 cles of the same strain, Hershey tried mixed infections with the r+ 

 and r to analyze the burst of single bacteria. He used a technique 

 already worked out by Dclbriick. After infection, he plated the in- 

 fected bacteria before the burst. Thus, each plaque on the plate 

 represented the phage particles coming from one bacterium, because 

 tlic burst takes place in the agar and remains concentrated at one 

 point. If the principle of mutual exclusion was correct, all the plaques 

 should have been either r or f+. Hershey had no idea how mixed 

 plaques would look. What he was planning to do was to sample some 

 plaques at random and to analyze the population obtained from each 

 plaque. But the plaques were neither of the r nor of the r+ tyP^- 

 They were mottled, r and r+ growing together to give a type of 

 plaque very different from either one alone. The actual presence of 

 the two types in the same plaque was confirmed. 



Mutual exclusion had been demonstrated in a very clear way for 

 morphologically and serologically unrelated strains like Tl and T2. 

 But, what would happen if similar strains were used, for instance T2 

 and T4, which have many features in common? Following Hershey's 

 lead, Delbriick found that T2 and T4 could grow together in the 

 same bacterium. 



In one experiment, Delbriick used, instead of T2, a strain of T2r. 

 The original experiment of Hershey with mixed infected bacteria 

 could be repeated, not using T2r and T2r+, but using instead T2r 

 and T4r+. At this stage something unexpected occurred. The bac- 

 teria infected with T2r and T4r+, besides yielding both infecting 

 types, yielded also two new types, T4r and T2r+. Recombination in 

 phage had been discovered. Not only could two different phage 

 particles grow in the same bacterium, but they could also recombine 

 some of their characters. At the Cold Spring Harbor Symposium 

 for Quantitative Biology in 1946 Delbriick and Bailey announced 

 this result in their paper, "Induced Mutations in Bacterial Viruses." 

 They used the following scheme to represent the new phenomenon: 



T4r+ > T4r (under the influence of T2r) 



At the end of their paper Delbriick and Bailey made the follow- 



