302 F. M. BURNET 



the serological character of its type are to some extent interchangeable 

 between related viruses, but as would be expected the "homozygous" complex 

 has a survival advantage over others, then we have the basis for a reasonable 

 interpretation of the findings. What is needed is a prolonged series of passages 

 of doubly neutralized material to obtain a stable serological recombinant or 

 to prove that it is impossible. 



The alternative interpretation has been proposed that the essential effect 

 is merely aggregation of virus particles of the two types. Reasonable ad hoc 

 assumptions that this is more likely to occur in a doubly infected cell than in 

 a simple mixture and that neutralization of one component may have an 

 effect of interference or inhibition on the other when the complex enters a 

 cell, would allow an equally satisfactory explanation. 



As in the case of influenza virus, more meaningful results may be obtainable 

 if markers other than serological ones can be utilized. A start in this direction 

 has been made by Vogt et at. (1957) who obtained d (delayed) mutants of a 

 type 1 poliovirus. These have lower pathogenicity for the monkey and on 

 slightly acid pH 6.8 medium show a marked delay in the appearance of 

 plaques on monkey kidney monolayers. Another heat-resistant mutant type 

 has been obtained, t 2 1 2 , which on crossing with the wild type gives clones of 

 intermediate heat resistance (Dulbecco, 1957). Further reports of this work 

 are awaited with much interest. 



C. Genetic Interaction with Psittacosis Virus 



Gordon and Mamay (1957) have demonstrated interaction between two 

 strains of psittacosis virus marked by specific drug resistances. One obtained 

 by Golub (1948) was resistant to sulfadiazine SA-r, the second to chlortetra- 

 cycline Ctc-r. Each was fully susceptible to the other drug. The experimental 

 method was to grow a mixture of SA-r and Ctc-r in embryonated eggs 

 protected by both drugs in amount adequate to protect against either strain 

 alone. The embryos were protected beyond the usual period but late deaths 

 occurred and from these doubly resistant clones of virus .were regularly 

 isolated. Since the development of drug tolerance normally requires a 

 prolonged series of passages, the combination of the two types of drug 

 resistance after a single passage points strongly to genetic interaction. 



D. The Berry-Dedrick Phenomenon — Transformation ofShope Fibroma Virus to 



Myxoma Virus 



In 1936, Berry and Dedrick described a phenomenon which probably 

 represents the first instance of genetic interaction between two viruses. Their 

 experiments were stimulated by those of Griffiths (1928) on the transforma- 

 tion of pneumococcal type by inoculation of mice with a live R strain of one 



