Caryophyllales are the main hosts of a unique set of ectomycorrhizal fungi in a Neotropical dry forest.

The ectomycorrhizal symbiosis was long thought to be restricted to temperate forests. However, as tropical forests have been explored, it has become clear that these habitats host unique ectomycorrhizal (ECM) fungi. We have been exploring tropical dry forests (TDF), which are endangered terrestrial ecosystems and hotspots of endemism. Since Fabaceae is the main plant family in this environment, we hypothesized that trees in this lineage would be the main ECM hosts. We sequenced the ITS rDNA region from fungi and both rbcL and trnL cpDNA from plants to identify both symbiotic partners from root tips. The systematic position of each symbiont was confirmed by Bayesian phylogenetic inference. We identified 20 plant species belonging to 10 families that hosted 19 unique ECM fungal species from 5 lineages. Most ECM fungi were associated with Caryophyllales, not with Fabaceae. Achatocarpus and Guapira, the main hosts, are scattered throughout the forest and are not in monodominant patches. The low ECM fungal diversity can be explained by the low density of host plants and their high specificity. Our results indicate that Caryophyllales is an important order of tropical ECM hosts with at least four independent evolutionary lineages that have evolved the ability to form ectomycorrhizae.