Multi-year time shift study of bacteria and phage dynamics in the phyllosphere

Coevolutionary dynamics shape diversity within and among populations but are difficult to study directly. Time shift experiments, where populations of species A from one point in time are experimentally challenged against populations of species B from past, contemporary, and/or future time points, and vice versa, are a particularly powerful tool to measure coevolution. This approach has been primarily applied to study host-parasite interactions and proven useful in directly measuring coevolutionary change and distinguishing among coevolutionary models. However, these data are only as informative as the time window over which they were collected, and data from shorter coevolutionary windows might conflict with data collected over longer time periods. Previous work on natural microbial communities from horse chestnut tree leaves used time shifts to uncover an apparent asymmetry, whereby hosts tended to be resistant to phages from all earlier points in the growing season while phages tended to be most infective on hosts from the recent past. Here we extend the time window over which these infectivity and resistance ranges are observed from within a growing season to across years and confirm that the previously observed asymmetry holds over longer timescales.