Experimental phylogeny of neutrally evolving DNA sequences generated by a bifurcate series of nested polymerase chain reactions.

A known phylogeny was generated using a four-step serial bifurcate PCR method. The ancestor sequence (SSU rDNA) evolved in vitro for 280 nested PCR cycles, and the resulting 15 ancestor and 16 terminal sequences (2,238 bp each) were determined. Parsimony, distance, and maximum likelihood analysis of the terminal sequences reconstructed the topology of the real phylogeny and branch lengths accurately. Divergence dates and ancestor sequences were estimated with very small error, particularly at the base of the phylogeny, mostly due to insertion and deletion changes. The substitution patterns along the known phylogeny are not described by reversible models, and accordingly, the probability substitution matrix, based on the observed substitutions from ancestor to terminal nodes along the known phylogeny, was calculated. This approach is an extension of previous studies using bacteriophage serial propagation, because here mutations were allowed to occur neutrally rather than by addition of a mutagenic agent, which produced biased mutational changes. These results provide for the first time biochemical experimental support for phylogenies, divergence date estimates, and an irreversible substitution model based on neutrally evolving DNA sequences. The substitution preferences observed here (A to G and T to C) are consistent with the high G+C content of the Thermus aquaticus genome. This suggests, at least in part, that the method here described, which explores the high Taq DNA polymerase error rate, simulates the evolution of a DNA segment in a thermophilic organism. These organisms include the bacterial rod T. aquaticus and several Archaea, and thus, the method and data set described here may well contribute new insights about the genome evolution of these organisms.