Not only transitions in nutritional modes but also niche shifts facilitate mycorrhizal fungal specialization in Burmannia

Mycoheterotrophs are non-photosynthetic plants that obtain all of their carbon requirements through parasitizing mycorrhizal fungi. They originated from the autotrophic ancestors and usually have more specific relationships with fungi than that of green plants, for reasons that are largely unknown. Determining the factors that lead to specificity in mycoheterotrophs could provide insights on the constraints to mycoheterotrophic evolution.Here we assess the fungal diversities in mycoheterotrophic plants and their co-occurring plants to determine the roles of ecological factors on the specific fungal associations in mycoheterotrophic plants. We investigated mycorrhizal fungal communities in 16 populations of seven Burmannia species with different trophic modes and their co-occurring plants using high-throughput sequencing to assess the tripartite relationships of fungi, mycoheterotrophs and co-occurring autotrophs.We found that mycoheterotrophic species have similar fungal richness to their chlorophyllous relatives, indicating that they are not associated with a reduced set of fungal partners. The preference of mycoheterotrophic species toward specialized fungal assemblages is consistent with the pattern found in the green autotrophic plants within forest habitats, suggesting a coupling of the fungal phylogenetic constraints between mycoheterotrophs and their co-occurring autotrophs. We furthermore show that the turning to fungal communities having closer phylogenetic relationships during habitat shifts from open grasslands to shaded forests might provide the basis for the specialization of mycorrhizal associations in mycoheterotrophic species of Burmannia.Our findings suggest that fungal niche shifts may have promoted fungal partner changes and specialization in mycoheterotrophic plants. Our results provide new insights into the ecological factors leading to the specialized interactions between mycoheterotrophic plants and their fungal partners, expanding our understanding of the evolutionary trajectories followed by mycoheterotrophs.