INTERFERENCE BETWEEN ANIMAL VIRUSES 175 



period preceding viral increase is shortened in eggs infected with, large doses 

 of LEE (Liu and Henle, 1951b). 



Other systems involving interference between active myxoviruses in the 

 allantoic cavity (see Table I) must be interpreted in the light of these subtle 

 quantitative requirements. This applies particularly to the apparent conflict 

 concerning interference between influenza and mumps viruses. Ginsberg and 

 Horsfall (1949) claimed that these two agents were not mutually exclusive. 

 Actually their experiments showed significant reciprocal suppression of 

 multiplication, an inference borne out by subsequent work by Isaacs and 

 Edney (1950a) and by the Henles (see Henle, 1950). 



b. Homologous Systems. The interplay of interference and genetic interac- 

 tion in mixed infection with two homotypic strains of influenza virus differing 

 in strain-specific antigens or virulence and other markers was first demon- 

 strated by Burnet and Edney (1951) and Burnet and Lind (1951a,b) with 

 MEL and NWS in mouse brain. In mixed infections of eggs, the quantitative 

 principle was similar to that described for heterologous pairs in that distinct 

 advantage of one over the other strain resulted in suppression of the latter. 

 When both were given in dynamically equivalent amounts, the yield contained 

 a high proportion of recombinant particles (Hirst and Gotlieb, 1953a). In 

 contrast to the situation for heterologous pairs, Gotlieb and Hirst (1954) have 

 offered evidence suggesting that the genetic compatibility of homologous 

 pairs, e.g., MEL and WS, expresses itself in that recombinants are not merely 

 phenotypically mixed but may behave as heterozygous or diploid particles 

 as well. Are the methods commonly used for detecting phenotypic or genetic 

 recombinants sufficiently sensitive to reveal their presence under unbalanced 

 circumstances, where one strain has a decided quantitative advantage over 

 the other? In other words, is the phenotypic suppression of one strain under 

 such circumstances really an expression of interference in the strict sense 

 (mutual exclusion) or of decided genetic or phenotypic dominance of one 

 parent type in recombinant progeny? The evidence at hand does not provide 

 an answer. 



3. Interference and Genetic Interaction between Inactivated and Active 

 Myxoviruses 



The interfering capacity of influenza viruses is retained after partial or 

 complete abolition of infectivity by UV-irradiation (Henle and Henle, 1943; 

 Ziegler et al., 1944; Powell and Setlow, 1956), ionizing radiation (Powell and 

 Pollard, 1956), heat or formaldehyde (Henle and Henle, 1943, 1944a; Isaacs 

 and Edney, 1950a), or sulfur mustard (Fong and Louie, 1953). Earlier data 

 by Henle and Henle (1947) on the UV-sensitivity of the interfering component 

 of the viral particle have been supplemented by studies of Powell et al. 

 (Powell and Pollard, 1956; Powell and Setlow, 1956) on sensitivity to ionizing 



