70 



ARTHUR C. UPTON 



(AKR X EF)Fi hybrids were compared with those in the two parental strains, 

 AKR and RF. Preliminary results (Table III) show the incidence of spon- 

 taneous and induced leukaemia in the F^ hybrids to be intermediate between 

 that of the parental strains. The data are consistent with the possibility that 

 the same leukaemogenic agent (1) is present in both AKR and RF strains, 



-5 5 



1 

 Birth 



25 50 



Age at irradiation (days) 



75 



Fig. 3. Incidence of myeloid leukaemia in X-irradiated RF mice as influenced by age. 

 (300 r, 250 kVp, whole-body X-irradiation.) The range of incidence in non-irradiated controls 

 is shown by the cross-hatched band. (From Upton et ah, 1960.) 



(2) produces different leukaemias owing to strain differences in host respon- 

 siveness, and (3) is transmitted vertically to F^ hybrids, the hybrid's response 

 being intermediate because of genetic heterozygosity. Although such an 

 interpretation remains to be proven, the vertical transmission of leukaemia 

 viruses is well documented (see Gross, 1961). The extent of vertical trans- 

 mission may also vary, however, depending on the virus and the host strain 

 (Law and Moloney, 1961). Thus, the observed differences between parental 

 strains and hybrids may conceivably reflect differences in transmission of a 

 common agent as well as differences in susceptibility to such an agent. This 

 possibility could be tested using viral preparations of known potency, such 



