EXPERIMENTAL MOLECULAR EVOLUTION OF BACTERIOPHAGE T7.

Abstract

We present an analysis of molecular evolution in a laboratory-generated phylogeny of the bacteriophage T7, a virus of 40 kilo-base pairs of double-stranded DNA. The known biology of T7 is used in concert with observed changes in restriction sites and in DNA sequences to produce a model of restriction-site convergence and divergence in the experimental lineages. During laboratory propagation in the presence of a mutagen, the phage lineages changed an estimated 0.5%-1.5% in base pairs; most change appears to have been G → A or C → T, presumably because of the mutagen employed. Some classes of restriction-site losses can be explained adequately as simple outcomes of random processes, given the mutation rate and the bias in mutation spectrum. However, some other classes of sites appear to have undergone accelerated rates of loss, as though the losses were selectively favored. Overall, the wealth of knowledge available for T7 biology contributes only modestly to these explanations of restriction-site evolution, but rates of restriction-site gains remain poorly explained, perhaps requiring an even deeper understanding of T7 genetics than was employed here. Having measured these properties of molecular evolution, we programmed computer simulations with the parameter estimates and pseudo-replicated the empirical study, thereby providing a data base for statistical evaluation of phylogeny reconstruction methods. By these criteria, replicates of the experimental phylogeny would be correctly reconstructed over 97% of the time for the three methods tested, but the methods differed significantly both in their ability to recover the correct topology and in their ability to predict branch lengths. More generally, the study illustrates how analyses of experimental evolution in bacteriophage can be exploited to reveal relationships between the basics of molecular evolution and abstract models of evolutionary processes.