Posy E. Busby

Tree genotype mediates covariance among communities from microbes to lichens and arthropods

Summary 1. Community genetics studies frequently focus on individual communities associated with individual plant genotypes, but little is known about the genetically based relationships among taxonomically and spatially disparate communities. We integrate studies of a wide range of communities living on the same plant genotypes to understand how the ecological and evolutionary dynamics of one community may be constrained or modulated by its underlying genetic connections to another community. 2. We use pre-existing data sets collected from Populus angustifolia (narrowleaf cottonwood) growing in a common garden to test the hypothesis that the composition of pairs of distinct communities (e.g. endophytes, pathogens, lichens, arthropods, soil microbes) covary across tree genotypes, such that individual plant genotypes that support a unique composition of one community are more likely to support a unique composition of another community. We then evaluate the hypotheses that physical proximity, taxonomic similarity, time between sampling (time attenuation), and interacting foundation species within communities explain the strength of correlations. 3. Three main results emerged. First, Mantel tests between communities revealed moderate to strong (q = 0.25‐0.85) community‐genetic correlations in almost half of the comparisons; correlations among phyllosphere endophyte, pathogen and arthropod communities were the most robust. Secondly, physical proximity determined the strength of community‐genetic correlations, supporting a physical proximity hypothesis. Thirdly, consistent with the interacting foundation species hypothesis, the most abundant species drove many of the stronger correlations. Other hypotheses were not supported. 4. Synthesis. The field of community genetics demonstrates that the structure of communities varies among plant genotypes; our results add to this field by showing that disparate communities covary among plant genotypes. Eco-evolutionary dynamics between plants and their associated organisms may therefore be mediated by the shared connections of different communities to plant genotype, indicating that the organization of biodiversity in this system is genetically based and non-neutral.

Leaf endophytes and Populus genotype affect severity of damage from the necrotrophic leaf pathogen, Drepanopeziza populi

Fungal leaf endophytes—nonpathogenic microfungi that live within plant leaves—are ubiquitous in land plants. Leaf endophytes and host plant genotypes may interact to determine plant disease severity. In a greenhouse inoculation experiment, we found that leaf endophyte species and Populus angustifolia genotypes both affected disease outcomes in plants inoculated with the necrotrophic leaf pathogen Drepanopeziza populi. Contrary to many studies showing endophytes conferring defense, all plant genotypes inoculated with the endophyte Penicillium sp. prior to inoculation with the pathogen D. populi were characterized by greater pathogen symptom severity than plants inoculated with the pathogen only. We quantified defense gene expression via qRT–PCR, but found no evidence that increased pathogen damage was related to differential expression of the assayed genes. A second endophyte, Truncatella angustata, which was previously found to reduce symptom severity of the biotrophic pathogen Melampsora in Populus trichocarpa, did not affect symptom severity of the necrotrophic pathogen D. populi or defense gene expression. Overall, our study highlights the variable effects of endophytes on pathogen symptom severity, and illustrates that plant genotypic variation can remain important for disease outcomes even in the presence of endophytes altering disease. Additional work is needed to elucidate the mechanism by which fungal leaf endophytes alter disease in their host plants.

Genetic basis of pathogen community structure for foundation tree species in a common garden and in the wild

Summary 1. Genetic variation within and among foundation plant species is known to affect arthropod, plant and soil microbial communities. We hypothesized that the same would be expected for pathogen communities, which have typically been studied only as individual pathogen species. 2. In a common garden in Utah, USA, we first tested how genetic differences within and among Populus angustifolia, P. fremontii and their interspecific hybrid P. 9 hinckleyana affect a fungal leaf pathogen community. Next, we tested how Populus genetic differences at the level of species and hybrids affect fungal leaf pathogen communities in the wild, specifically in a natural Populus hybridization zone (13 river km) and throughout the larger Weber River watershed (150 river km). 3. In the common garden, genetic variation both within and among Populus species and hybrids significantly affected the structure (i.e. species abundances and composition) of pathogen communities. In the wild, genetic variation among Populus species and hybrids also significantly affected pathogen communities, though not as strongly as was found in the common garden environment. Stand-level density of the susceptible Populus species most strongly affected the structure of pathogen communities in the hybrid zone. 4. Synthesis. Plant species and genotypic variation can affect the local and geographic distribution of pathogen communities in a similar fashion as other diverse organisms (e.g. arthropods, plants, soil microbes), both within a relatively controlled common garden environment and in the wild.