Genetic resistance to Friend leukemia virus in mice: masking of Fv-2 phenotype by an epistatic gene, Fv-4.

Mice of strain G which have Fv-4rr genotype resemble those with Fv-2rr genotype in resisting the induction of spleen foci or splenomegaly by Friend leukemia virus. However, NB-tropic Friend virus replicates well in Fv-2rr mice, but not in G mice. To determine the Fv-2 genotype, G mice were crossed with Fv-2rr mice, and the hybrids were tested for the susceptibility to NB-tropic Friend virus. The (G X Fv-2rr) F1 hybrids were similar to G. In F1 X Fv-2rr backcross generation, three types of mice appeared; 1) G type, 2) Fv-2rr type and 3) Fv-2ss type. Mice of the last type supported virus replication and developed splenomegaly. Mice of types 1), 2) and 3) appeared with the expected ratio of 2:1:1. It will be concluded that in G mice Fv-4r gene suppresses the virus replication, and hence masks the expression of Fv-2s gene. Data of F2 generation support this conclusion.