MUTATION AND PHENOTYPIC VARIATION IN PHAGES 305 



(1922), Asheshov (1924), Burnet (1933d), and Asheshov, 

 Asheshov, Khan, and Lahiri (1933). 



Some of the earliest observations of mutation affecting plaque 

 type and host range were made by Sertic (1929b, c). Small 

 plaque mutants in T2 were described by Hershey and Rotman 

 (1949) and in T4 by Doermann and Hill (1953). The use of 

 nutrient agar containing water soluble dyes to recognize "color 

 mutations" in Tl was introduced by Bresch (1953). Several 

 plaque-type mutants of T4 are illustrated in Figure 9. 



/. Mutations Affecting Stability 



The stability of phages subjected to inactivating agents can be 

 measured with precision (Chapters V and VI). It ought, 

 therefore, to be a simple matter to isolate and recognize resistant 

 mutants. However, this procedure has more often failed than 

 succeeded. 



One difficulty was discovered by Adams and Lark (1950). 

 Phage T5 yields heat stable mutants but their presence in a stock 

 is usually obscured by the much more numerous nonmutant 

 heat-stable particles (Adams, 1953a). 



Mutants selected for altered adsorption characteristics are 

 often found to differ from the wild stock in sensitivity to heat 

 (Burnet and Freeman, 1937; Hershey and Davidson, 1951; 

 Streisinger and Franklin, 1956). 



T2 and T4 differ in resistance to ultraviolet light and this 

 difference can be traced to a single locus by interspecific crosses 

 (Streisinger, 1956a). Attempts to isolate a mutant of T2 having 

 the resistance of T4 have not succeeded, however. 



A host range mutant of phage CI 6 shows an increased sus- 

 ceptibility to inactivation by formaldehyde (Wahl and Blum- 

 Emerique, 1950). 



g. Mutations Affecting Antigenic Specificity 



The serological difference between T2 and T4 segregates in 

 interspecific crosses with the host range character, both being 



