UC Santa Cruz

The rare species advantage postulates that dominant plant species experience greater disease pressure, as their abundance can support the spread of specialized pathogens. Yet most pathogens are not host specific – pathogen host ranges frequently encompass multiple closely related plant species, resulting in increased sharing of pathogens between closely phylogenetically related community members. We examined the role host phylogenetic rarity plays in pathogen spread in a diverse plant community by inserting novel hosts (non-native plant species newly introduced into a novel habitat) into a coastal grassland and identifying the patterns of pathogen spillover from resident plant species onto the novel hosts. We identified fungi present in leaf tissues of novel hosts via metagenomic sequencing of the fungal internal transcribed spacer (ITS) region to generate operational taxonomic units (OTUs). We found that 161/342 (47.1%) OTUs identified were singletons. Excluding singletons, the median number of novel plant species colonized by a given OTU was 4, and the mean was 11.5, while 19/181 (10.5%) of non-singleton OTUs were found on 14 (50%) or more novel hosts. The proportion of broad generalist and rare fungal taxa on novel hosts were similar to several other systems, including other grasslands, temperate and boreal forests, tropical rainforests, wetlands, and deserts. We did not find a relationship between phylogenetic relatedness of a novel host to the surrounding vegetation and the fungal species richness on that host. Weighting phylogenetic distance by local plant species abundance did not strengthen the relationship between phylogenetic distance and fungal richness on novel hosts.