When good relationships go bad.

ycorrhizae are ancient, widespread associations between fungi and the roots of many species of plants. In these symbioses, the plants supply carbohydrates (the products of photosynthesis) to their fungal partners, which reciprocate by facilitating the uptake of mineral nutrients from the soil. In a reversal of the usual relationship , some non-photosynthetic plants — termed epiparasites — obtain carbohydrates from mycorrhizal fungi that are also associated with photosynthetic plants in the immediate environment. In other words, epiparasites feed off green plants in their communities, and they do so via a fungal 'bridge'. A barrier to understanding the ecology of mycorrhizal plants, including epiparasites, has been the difficulty of identifying the fungi with which they are associated. There are two main types of mycorrhizae, ectomycorrhizae (ECM) and arbuscular mycorrhizae (AM). The first type is produced by mushroom-forming fungi — including choice edibles such as truffles and chanterelles. The second is produced by soil fungi that belong to the taxonomic order Glomales. These fungi produce no above-ground structures in their life cycle, and reproduction is accomplished by large, multinucleate spores that are produced underground and on which the taxonomy of the group has traditionally been based (Fig. 1a). Until now, all documented cases of epi-parasitism have involved ECM fungi. This is surprising, because AM symbioses involve roughly 70% of all plant species — including many agricultural crops — and are ecologically much more widespread than ECM symbioses, which involve only about 30 plant families, mostly trees. On page 389 of this issue, Bidartondo et al. 1 provide the first molecular evidence that the fungal partners of two distantly related groups of epipara-sites are AM fungi. Bidartondo et al. used the polymerase chain reaction to amplify fungal ribosomal genes (rDNA) from the roots of epiparasites in the Corsiaceae (a group related to lilies) and the Gentianaceae (gentian family). Anatomical studies in these groups had suggested that AM fungi are present, but gave no clues to their precise identity. Bidartondo et al. generated sequences of the fungal rDNAs and compared them to a database of Glomales rDNA sequences. Evolutionary analyses of the rDNA sequences provided the necessary taxonomic resolution, and showed that both groups of epiparasites are highly specific for particular species of Glomales. In the most intensively sampled species, Arachnitis uniflora (Corsiaceae; Fig. 1b), Bidartondo et al. found that eight plants in three populations were all associated with a single fungal species. Neighbouring …