Evolution of bacteriophage T7 in a growing plaque.

The emergence of mutants during the 10(9)-fold amplification of a bacteriophage was spatially resolved in a growing plaque. When wild-type phage T7 was grown on an Escherichia coli host which expressed an essential early enzyme of the phage infection cycle, the T7 RNA polymerase, mutant phage relying on this enzyme appeared in 10(8) phage replications and outgrew the wild type. Spatial resolution of the selection process was achieved by analyzing stab samples taken along a plaque radius. Different mutants were selected at different rates along different radii of the plaque, based on host range and restriction patterns of the isolates. The mutants deleted up to 11% of their genomes, including the gene for their own RNA polymerase. They gained an advantage over the wild type by replicating more efficiently, as determined by one-step growth cultures.