American Zoologist
2001: 40(6): 956.

Interactions of ectomycorrhizal fungi and ectomycorrhizal epiparasites

Bruns, T. D.1, M.I. Bidartondo2 and D.L. Taylor1

1 Department of Plant and Microbial Biology, 321 Koshland Hall, University of California, Berkeley, California, 94720, USA
2 University of California at Berkeley, Department of Environmental Science, Policy and Management, 321 Koshland Hall, Berkeley, California 94720-3102.

We have studied non-photosynthetic plants in the Monotropoideae and Orchidaceae that obtain their fixed carbon from surrounding photosynthetic plants through shared ectomycorrhizal fungi. These "epiparasites" are only indirectly connected to surrounding trees, which are the ultimate source of their carbon; their proximal hosts are fungi that are mutualistically associated with the trees. We found that all epiparasitic plants have specific fungal associations, which in some cases exhibit geographic mosaic patterns. In two species, Sarcodes sanguinea and Pterospora andromedea, seed germination is stimulated by diffusible compounds from the fungi associated with the adult plants, but not by distantly related fungi. Thus, the first step in establishment of these specific interactions is based on a specific plant reaction to the presence of its fungal host. This fact, coupled with localized seed dispersal, has parallels with insect host-fidelity systems and may facilitate host-race formation, which we now have evidence for in Corallorhiza species. In nature, S. sanguinea appears to increase the presence of its host fungus, Rhizopogon ellenae, and the abundance of Abies magnificaroots to which the latter is associated. This either provides an unusual case of parasite-induced host-hypertrophy, or alternatively it suggests an unexpected mutualistic aspect to this symbiosis.

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