AnalysesWe determined the population size of the bacteriophage by adding 30ml of chloroform to a1 ml subsample of each chemostat (to remove bacteria) and plating 100ml of the purifiedsubsample of bacteriophage with 300ml of an overnight culture of bacteria in 3 ml of softagar. We counted the number of plaques (spots cleared of bacteria by the bacteriophage)after 24 h. Becau se the bacteriophage were well mixed before plating, each plaque waspresumed to result from a single viral particle. A lawn of ancestral bacteria (B0) was used todetermine the total size of the bacteriophage population and a lawn of resistant bacteria(B1) was used to calculate the number of host-range mutants (T71) in the bacteriophagepopulation.We determined the population size of the bacteria by plating 100ml of a subsample(without chloroform treatment) and counting the number of bacterial colonies presentafter 24 h of incubation. To determine the size of the B1population, we plated the bacteriawith an equal volume of ancestral bacteriophage.To investigate local adaptation in the bacteriophage through time, we isolated two tothree colonies of B1from the same day (day 9, 13 and 19) from each treatment during eachrun of the experiment. For each adaptation assay, overnight cultures of the colonies weregrown up in 1 mg ml21glucose media. We plated replicate samples of T71from each timepoint on a lawn of sympatric (from the same chemostat) and allopatric (from a differentchemostat, but at the same productivity level) B1from the overnight culture. We used the‘efficiency of plating’ (the number of plaques on each host) as a measure of bacteriophageinfectivity.We calculate d adaptation as the ratio of the number of plaques formed by thenumerically dominant bacteriophage on the dominant sympatric host to the number ofplaques formed on the allopatric host. Ratios of the number of plaques formed on thebacterial isolates from each replicate run of the experiment were averaged to give a meanratio for each run. Only one replicate run of the experiment was used in assays of theclosed/high productivity community due to contamination. There were often no plaqueson allopatric hosts, therefore we coded the data by adding one to bo th the numerator andthe denominator to allow calculation of a ratio. A value above one indicates localadaptation and a number below one indicates local maladaptation (that is, the number ofplaques was higher on the allopatric host than on the sympatric host). All ratios were log-transformed before analysis.Variation in adaptation was assessed by calculating the coefficient of variation of theadaptation ratios. Significance was tested using an F-test, with a sequential Bonferronicorrection for five pairwise comparisons25. 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