Bacteria-phage interactions across time and space: merging local adaptation and time-shift experiments to understand phage evolution.

The study of parasite local adaptation to host populations offers important insight into the spatial scale of host-parasite interactions. For parasites adapting to local hosts, the process is continually driven by change in the host population, in response to either the parasite or alternative selection pressures. In the case of reciprocal coevolutionary change, this adaptation should generate a pattern whereby parasites are most fit against hosts from the recent past (which have not yet responded to parasite-mediated selection) and least fit against future host populations (with increased resistance). I argue that combining data on local adaptation across space with data on evolutionary responses over time can offer novel insight into the process of adaptation. Using bacteriophages from horse chestnut trees, I compare infectivity on bacterial hosts isolated from either the same tree or different trees over multiple months of the growing season and find that phage adaptation to local hosts is most pronounced on bacterial hosts from the recent past. These results confirm that phages are well adapted to bacterial populations living within eukaryotes and more broadly suggest that local adaptation studies may underestimate the magnitude of parasite evolution, as host and parasite adaptation are confounded within contemporary time points.