Temperature-sensitive () mutants obtained by undiluted passages of fowl plague virus at 33 °C have their defects located mainly in RNA segments 3, 4 and 8 as determined by rescue to wild-type with standard mutants. This result is different from that obtained after treatment of virus with mutagens, where the frequency of mutations follows roughly the target size of the RNA segments. Many isolates generated after undiluted passages at 33 °C, which seem to have mutations in RNA segments 3 and 4, can be rescued to wild-type. This occurs, however, with certain defined standard mutants having a defect in RNA segment 4, but not by other segment 4 mutants. One such mutant, 1/93 ( defect in segment 3), interferes with the multiplication of 227 ( defect in segment 4) at the permissive temperature, presumably at the level of vRNA synthesis, preventing reassortment to wild-type. Similarly, 263 ( defect in segment 3) interferes with the multiplication of 1/1 ( defect in segment 4). For other such interfering mutants, the mechanism preventing reassortment to wild-type is different from that of 1/93 or 1/1, but is not yet understood. Thus, the number of mutations as determined by rescue with standard mutants in isolates obtained by undiluted passages is overestimated due to intrinsic interference.


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