Dispersal overwhelms variation in host quality to shape nectar microbiome assembly.

O_LIEpiphytic microbes frequently impact plant phenotype and fitness, but effects depend on microbe community composition. Deterministic filtering by plant traits and dispersal-mediated processes can affect microbiome assembly yet their relative contribution is poorly understood. C_LIO_LIWe tested the impact of host-plant filtering and dispersal limitation on nectar microbiome abundance and composition. We inoculated bacteria and yeast into 30 plants across 4 phenotypically distinct cultivars of Epilobium canum. We compared the growth of inoculated communities to openly visited flowers from a subset of the same plants. C_LIO_LIThe abundance and composition of microbial communities differed among plant individuals and cultivars in both inoculated and open flowers. However, plants hosting the highest microbial abundance when inoculated did not have the highest abundances when openly visited. Rather microbial density among open flowers was correlated with pollen receipt, a proxy for animal visitation, suggesting a primary role of deterministic dispersal in floral microbiome assembly despite variation in host-quality. C_LIO_LIWhile host-quality can affect microbiome assembly, variation in dispersal was more important here. Host quality could drive microbial community assembly in plant tissues where species pools are large and dispersal is consistent, but dispersal may be more important when microbial dispersal is limited, or arrival order is important. C_LI