GENETIC INTERACTIONS BETWEEN ANIMAL VIRUSES 



303 



type and heat-killed smooth organisms of another. The analogous experiment 

 was to equate fibroma virus with the E, pneumococcus and myxoma virus 

 with the more virulent smooth form. Mixtures of extracts from fibroma and 

 myxoma lesions were prepared, in which the fibroma virus was untreated and 

 the myxoma extract heated to a temperature, which might be anywhere 

 between 56 and 85°C. (Berry, 1937), that completely inactivated it. A sub- 

 stantial proportion of such inoculated rabbits developed myxoma lesions. 



California!! 



/ 



Standard 

 laboratory 



Australian 

 field 



/ 

 / 



Neuromyxoma 



/ 



Standard 

 OA fibroma 



/ 



/ 



French 



European 

 attenuated 



Boerlage 

 fibroma 



/ 



/ 



1A Fibroma 



Increasing tumor production > 



Fig. 1. Myxoma-fibroma relationship. 



This observation has been repeatedly confirmed (Gardner and Hyde, 1942; 

 Shope, 1950; Hurst, 1937; Smith, 1952) but all workers have found exasperat- 

 ing irregularities in the results. 



The most extensive recent account is that of Smith (1952). She points out 

 that the viruses of the myxoma-fibroma group show a wide range of virulence 

 and a tendency for dissociation of tumor-producing capacity and virulence. 

 If we combine Fenner and Marshall's (1957) subsequent Australian studies, 

 we can present a two-dimensional arrangement of strains according to tumor- 

 producing power and virulence (Fig. 1). A particularly interesting point is 

 that in the fibroma series at least prolonged storage in glycerol produces an 

 inheritable downward shift in the type of pathogenic activity, the end point 



