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Abstract Gene flow into populations can increase additive genetic variation and introduce novel beneficial alleles, thus facilitating adaptation. However, gene flow may also impede adaptation by disrupting beneficial genotypes, introducing deleterious alleles, or creating novel dominant negative interactions. While theory and fieldwork have provided insight as to the effects of gene flow, direct experimental tests are rare. Here, we evaluated the effects of gene flow on adaptation in the nematode Caenorhabditis elegans during exposure to the bacterial parasite Serratia marcescens. We evolved hosts against non-evolving parasites for ten passages while controlling host gene flow and source population. We used source nematode populations with three different genetic backgrounds (one similar to the sink population and two different) and two evolutionary histories (previously adapted to S. marcescens or naïve). We found that populations with gene flow exhibited greater increases in parasite resistance than those without gene flow. Additionally, gene flow from adapted populations resulted in greater increases in resistance than gene flow from naïve populations, particularly with gene flow from novel genetic backgrounds. Overall, this work demonstrates that gene flow can facilitate adaptation, and suggests that the genetic architecture and evolutionary history of source populations can alter the sink population’s response to selection. 

Host populations often evolve defenses against parasites due to the significant fitness costs imposed by infection. However, adaptation to a specific parasite may alter the effectiveness of the host’s defenses in general. Consequently, the specificity of host defense may be influenced by a host population’s evolutionary history with parasites. Further, the degree of reciprocal change within an interaction may profoundly alter the range of host defense, given that antagonistic coevolutionary interactions are predicted to favor defense against specific parasite genotypes. Here, we examined the effect of host evolutionary history on host defense range by assessing the mortality rates of Caenorhabditis elegans host populations exposed to an array of Serratia marcescens bacterial parasite strains. Importantly, each of the host populations were derived from the same genetic background but have different experimental evolution histories with parasites. Each of these histories (exposure to either heat-killed, fixed genotype, or coevolving parasites) carries a different level of evolutionary reciprocity. Overall, we observed an effect of host evolutionary history in that previously coevolved host populations were generally the most susceptible to novel parasite strains. This data demonstrates that host evolutionary history can have a significant impact on host defense, and that host-parasite coevolution can increase host susceptibility to novel parasites.