Stable isotope analysis, field observations and synthesis experiments suggest that Odontia is a non-mycorrhizal sister genus of Tomentella and Thelephora

Ectomycorrhizal symbiosis has evolved multiple times in plants and fungi, but the trophic status of certain fungal groups remains poorly understood due to their unculturability or ambiguous interpretation of biotrophic associations. Combining field observations, molecular identification of root tips, synthesis experiments and analysis of stable isotopes, we address the lifestyle of Tomentella crinalis and another species closely related to T. fibrosa that represents a sister group to the ectomycorrhizal genera Tomentella and Thelephora. Based on molecular analyses these two and other related species are moved to the genus Odontia. In Odontia species, ectomycorrhizal associations were not observed in nature or in various synthesis experiments. Although Odontia species normally fruit in old forests, Odontia ferruginea has also been identified from a deep belowground mine. Unlike saprotrophs, Odontia spp. and ectomycorrhizal fungi were not enriched in 13C compared with their woody fruiting substratum, suggesting that wood is not their major energy source. In contrast to ectomycorrhizal fungi, Odontia species and saprotrophs were not enriched in 15N relative to their substratum. Taken together, we suggest that Odontia spp. are non-mycorrhizal, but their nutrition differs from typical wood-rotting Basidiomycota.