GENETIC INTERACTIONS BETWEEN ANIMAL VIRUSES 289 



For obvious reasons it is much simpler to obtain and isolate recombinants 

 of the serological type of the inactivated parent than the reciprocal recom- 

 binant serologically similar to the active parent. 



It is possible, however, to provide a selective environment for isolation of 

 WSE recombinants from the system, heat-inactivated M + /active WS — , 

 by inoculation of the chorioallantois and reisolation from the embryo lung. 

 Two such strains were isolated and fully characterized by Lind and Burnet 

 (1957c). One showed unduly low pathogenicity for the chick embryo. 



All workers in this field have found that treatment of the allantoic cells 

 with the inactive virus may be carried out one or more days before adding the 

 active virus, and still give rise to typical recombinants. This observation 

 seems to offer possibilities for analysis of the process of infection in the cell, 

 but our own unpublished experience was that, although recombinants are 

 regularly obtainable, the yield was very small and variable, so that the 

 system we used was unsuitable for critical experiments. 



A point of interest in the use of M -fc- and WS — strains for crosses in which 

 one component was inactivated at 37°C. is in regard to interference by the 

 inactive virus given 18 hours before de-embryonation and addition of the 

 active virus. Table III gives the findings of one set of experiments (Lind and 

 Burnet, 1957c). 



TABLE III 



Production of Recombinants and Interference 

 by Virus Inactivated by Thermal Degradation 



Inactivated Active T , c „ Recombinant 



, or .on v Interference ° . . , 



virus (3 i C.) virus yield. 



M+ WS- 160,200/240 10 1 



WS- M+ < 1/400 Nil 



Result shown as HA titer from combined infection/titer from infection with active 

 virus alone. 



In more extensive experiments a few WSE strains were isolated from heat- 

 inactivated WS — /active M + systems. The suggestion that the process of 

 interference prevents the genetic material being available for recombination 

 might lead to experiments of some importance. 



F. Redistribution of Virulence 



Some discussion has been already offered in the general section of this 

 contribution of the importance and difficulty of understanding the redistribu- 

 tion of virulence when virulent and avirulent strains are crossed. 



vol. in — 19 



