GENETIC INTERACTIONS BETWEEN ANIMAL VIRUSES 285 



Where other strains than these present additional characters they are 

 represented in similar fashion. For instance, two WS variants differ greatly in 

 the heat stability of their hemagglutinin. WSM is not destroyed at 65°C, 

 NWS is destroyed at 52°C. in 30 minutes. The qualities have been shown 

 as B+ and b~ respectively. Neuropathogenicity is symbolized by g, its absence 

 by 6. It must be stressed, however, that the symbols are primarily to repre- 

 sent phenotypic character differences between the two strains MEL and 

 WSE. Any extension may lead to unjustified assumption that similar 

 phenotypic characters are based on the same genetic mechanism as in the 

 MEL/WSE system. 



C. Interchange of Linkage Groups 



Very large numbers of recombination experiments between MEL and WSE, 

 and among various other strains, variants, and recombinants of influenza A 

 types, have now been carried out. Examination of the recombinants shows 

 that there is only a limited range of new patterns of characters and that with 

 a few exceptions these patterns correspond to an interchange of only two 

 linked groups of characters. This is most clearly seen from an analysis of the 

 recombinants obtained from the interaction of MEL and WSE (Burnet and 

 Lind, 1952): 



ABDF — CE + abclf — ce -» ABDF — ce + abdf — CE 

 MEL WSE M+ WS — 



The interaction is a reciprocal one. The primary recombinants (which we have 

 called M + and WS — ) can be repeatedly reisolated without change of 

 character and are stable virus types. They interact to give strains of MEL and 

 WSE character (Lind and Burnet, 1953): 



ABDF — ce + abdf — CE -> ABDF — CE + abdf — ce 

 When a neuropathogenic strain is used the quality g is associated with the 

 ce "linkage group," not with abdf. Neuro-MEL recombinants have the 

 formula ABDF — ceg and when such a strain is crossed with WS — a 

 neuro-WSE strain is obtained (Lind and Burnet, 1954): 



ABDF — ceg + abdf — CEG -> abdf — ceg 

 Although the great majority of interactions follow this rule there are excep- 

 tions, e.g., in some cases the interaction of a strain of a different serological 

 type CAM with WSE proceeded as follows: 



A C DFCE + abdfce -> A C D — fee + abd — FCE 



Here the character of mouse lung pathogenicity was in the second linkage 

 group. From their limited tests it is probable that the strains Wr and WSN 

 used by Baron and Jensen (1955) behaved similarly: 



Ab — FG + abfg -> Ab~f (g) + ab — FG 



