Frequency of spontaneous mutations in an avian hepadnavirus infection.

In this study, we measured the frequency of revertants of a cytopathic strain of the duck hepatitis B virus that bears a single nucleotide substitution in the pre-S envelope protein open reading frame, resulting in the amino acid substitution G133E. Cytopathic virus mixed with known amounts of a genetically marked wild-type virus was injected into ducklings. Virus outgrowth was accompanied by a coselection of wild-type and spontaneous revertants during recovery of the ducklings from the acute liver injury caused by death of the G133E-infected cells. The frequency of individual revertants in the selected noncytopathic virus population was estimated by determining the ratio of each revertant to the wild-type virus. Spontaneous revertants were found to be present at frequencies of 1 x 10(-5) to 6 x 10(-5) per G133E genome inoculated. A mathematical model was used to estimate that the mutation rate was 0.8 x 10(-5) to 4.5 x 10(-5) per nucleotide per generation.