Community composition and ecology of neotropical root-associated fungi [electronic resource]

Root-associated fungi (RAF) shape the interface between plants and soil. This ubiquitous group of organisms influences ecosystem processes such as nutrient cycling, and shapes plant community diversity and composition. Until recently, methodological difficulties have prohibited extensive study of RAF community composition. Thus, questions as to how tropical RAF communities vary between hosts, and across space and environmental conditions remain unanswered. In this dissertation, I present three studies that employ DNA metabarcoding to characterize the community composition of RAF in Neotropical forests. In Chapter 1 we employed a transect-based systematic survey of RAF to quantify relative contributions of host phylogeny and spatial distance to structuring RAF communities. In Chapter 2 we used a targeted sampling scheme that controlled for host phylogeny and location to compare diversity and composition of RAF associating with rare vs. common tree species. In Chapter 3 we investigated plant-RAF interaction patterns among plants in the diverse tribe Psychotrieae (Rubiaceae), and compared the relative influence of edaphic conditions, host phylogeny, and spatial distance in structuring RAF communities. Results show that RAF communities were structured by host phylogeny, such that RAF community similarity decreased with phylogenetic distance between hosts. This pattern persisted across the range of phylogenetic distances included in these studies, from closely related congeners to members of highly divergent host clades. Analyses were repeated with fungal communities pooled at deeper taxonomic levels, and by trophic mode. Patterns of host phylogenetic structure show that closely related host plants shared more phylogenetically similar root fungal microbiomes than distantly related host plants. Thus, core components of the RAF microbiome are conserved across the host plant phylogeny. Edaphic conditions also influenced RAF distributions, but geographic distance was generally a stronger predictor of RAF composition, consistent with the possibility that dispersal limitation structures RAF communities at fine spatial scales. Results v differed between fungi from different trophic modes; putative symbiotrophs, which were dominated by arbuscular mycorrhizal fungi, showed relatively weak host preference, while pathotrophs showed stronger host preference. Janzen-Connell processes involving distance-dependent mortality due to species-specific natural enemies likely maintain forest diversity by inhibiting conspecific regeneration beneath parent trees. Other studies have shown that RAF communities drive these processes, but have treated fungal communities as a black box in doing so. The present dissertation illuminates this black box. Results from each chapter consistently show that Neotropical RAF meet the prerequisites of host preference and dispersal limitation necessary to drive forest diversity maintenance processes under the Janzen-Connell hypothesis. Furthermore, common plant species supported less diverse RAF communities than rare host plants, especially for mutualistic symbiotrophs. This correlation between host abundance and RAF diversity is consistent with the idea that differences in Janzen-Connell processes between rare and common plants may be driven by differences in their RAF communities. The implications of uncovering emerging patterns in RAF community composition for plant ecology are discussed throughout, highlighting that RAF community ecology represents an important frontier in understanding Earth's biodiversity.

Keywords: Biology.
Advisor: Dirzo Rodolfo primary advisor. Thesis advisor
Boggs Carol L.
Fukami Tadashi
Peay Kabir