Detection of evolving viruses.

The spread of viruses on a homogeneous lawn of receptive hosts provides an opportunity to detect the dynamics of their evolution. We have previously found that when repeated virus passages are confined to the expanding perimeter of a growing plaque, the appearance and outgrowth of genetically diverse strains (all descended from the same parent strain) can be traced along different radii of the plaque. As a plaque grows, the random mutation and selection of new fast-growing strains reduce the roundness or circularity of the growing plaque. Here we have quantified such changes in growing plaques of bacteriophage T7 using a digital imaging system. We find that T7 populations not adapted for fast growth exhibit a broader diversity of growth rates than populations adapted for fast growth. These results provide a foundation for understanding how viruses exploit mutation and selection processes to persist in nature.