RATES OF SPONTANEOUS MUTATION AMONG RNA VIRUSES

Simple methods are presented to estimate rates of spontaneous mutation from mutant frequencies and population parameters in RNA viruses. Pub lished mutant frequencies yield a wide range of mutation rates per gen ome per replication, mainly because mutational targets have usually be en small and, thus, poor samples of the mutability of the average base . Nevertheless, there is a clear central tendency for lytic RNA viruse s (bacteriophage Qbeta, poliomyelitis, vesicular stomatitis, and influ enza A) to display rates of spontaneous mutation of almost-equal-to 1 per genome per replication. This rate is some 300-fold higher than pre viously reported for DNA-based microbes. Lytic RNA viruses thus mutate at a rate close to the maximum value compatible with viability. Retro viruses (spleen necrosis, murine leukemia, Rous sarcoma), however, mut ate at an average rate about an order of magnitude lower than lytic RN A viruses.