A high resolution view of adaptive events

Coadaptation between bacterial hosts and plasmids often involves a small number of highly reproducible mutations. Yet little is known about the underlying complex dynamics that leads to such a single “correct” solution. Observing mutations in fine detail along the adaptation trajectory is necessary for understanding this phenomenon. We studied coadaptation between E. coli and a common artificial plasmid, pBR322, in a continuous turbidostat culture. To obtain a high resolution picture of early adaptive events, we used a highly sensitive duplex sequencing strategy to directly observe and track mutations with frequencies undetectable with conventional methods. The observed highly reproducible trajectories are governed by clonal interference and show rapid increases in the frequencies of beneficial mutations controlling plasmid replication followed by a profound diversity crash corresponding to the emergence of chromosomal variants. To the best of our knowledge our study represents the first comprehensive assessment of adaptive processes at a very fine level of resolution. Our work highlights the hidden complexity of coadaptation, and provides an experimental and theoretical foundation for future studies.