Genetic recombinants and heterozygotes derived from endogenous and exogenous avian RNA tumor viruses

A selective technique based on infective centers is described for isolating host range recombinants of avian RNA tumor viruses. This method was exploited to obtain recombinants between the genome of chick helper factor (chf) and nondefective strains of Rous sarcoma virus (RSV), including temperature-sensitive mutants. Recombinants of chf with the defective Bryan strain RSV could not be isolated. A majority of nondefective RSV particles selected as carrying the chf host range also carried the parental RSV host range; i.e., were of dual host range, but the progeny segregated into parental or recombinant genotypes. These observations suggested that the particles were at least partially diploid or polyploid and represented unstable “heterozygotes.” Genotypic mixing was not evident in chf-negative cells which contain viral DNA but not viral RNA, suggesting that genetic reassortment occurs among RNA molecules. A model is proposed in which reassortment of independent genome segments may be converted into stable recombinants following provirus formation in the next replicative cycle.