Abstract:

The interaction between plants and pathogens is an important force shaping the ecology and evolution of plant populations. While the potential of pathogens to drive evolution in hosts has been widely recognized, empirical evidence linking pathogen pressure to host population genetic structure is limited. For pathogens to exert evolutionary pressure on their host plants, their impact must vary with host genetic identity. We explored host-specific infection by bacterial pathogens in natural populations of Arabidopsis thaliana from Indiana and Michigan. The presence and titer of three foliar bacterial pathogens (Pseudomonas viridiflava, P. syringae , and Xanthomonas campestris ) was recorded in order to assess pathogen toad. In the A. thaliana host, a set of 149 single nucleotide polymorphisms distributed across the genome was used to assess genetic variation.

Individual hosts and local populations show differences in bacterial infection rate and titer in the leaf, and all bacterial species reach densities with the potential for fitness effects on the host. All bacterial species are genetically diverse, and the presence of P. syringae is associated with the presence of P. viridiflava. There is also extensive genetic diversity in A. thaliana : this data set reveals deep divisions in the distribution of genetic diversity in the Midwest, and the presence of isolation by distance in the region. Nonetheless, several genome-wide haplotypes are shared by multiple plants, allowing the comparison of bacterial infection among host haplotypes. For P. viridiflava and X. campestris, the presence or absence of bacterial infection varies with the genome-wide haplotype of the host. While there is no association between host haplotype and the presence of the pathogen for P. syringae. density varies among highly replicated host haplotypes. Within P. viridiflava, the distribution of two genetically distinct clades also varies with host haplotype. While the relationship between host haplotype and P. viridiflava infection is affected by geographic structure in the distributions of the host and pathogen in addition to the haplotype of the host, there is a clear ecological basis for variable pathogen pressure on host haplotype in the interaction between A. thaliana and its bacterial pathogens.