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First published online August 19, 2009; doi:10.3732/ajb.0900058 American Journal of Botany 96: 1665-1675 (2009) © 2009 Botanical Society of America, Inc.

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Tuberculate ectomycorrhizae of angiosperms: The interaction between Boletus rubropunctus (Boletaceae) and Quercus species (Fagaceae) in the United States and Mexico¹

Farlow Herbarium of Cryptogamic Botany and Department of Organismic and Evolutionary Biology, Harvard University, 22 Divinity Avenue, Cambridge, Massachusetts 02138 USA

ABSTRACT

Tuberculate ectomycorrhizae (TECM) are unique structures in which aggregates of ectomycorrhizal roots are encased in a covering of fungal hyphae. The function of TECM is unknown, but they probably enhance the nitrogen nutrition and disease resistance of host plants. Trees in the Pinaceae form TECM with species of Rhizopogon and Suillus (Suillineae, Boletales). Similar tubercules are found with diverse angiosperms, but their mycobionts have not been phylogenetically characterized. We collected TECM in Mexico and the USA that were similar to TECM in previous reports. We describe these TECM and identify both the plant and fungal symbionts. Plant DNA confirms that TECM hosts are Quercus species. ITS sequences from tubercules and sclerotia (hyphal aggregations that serve as survival structures) matched sporocarps of Boletus rubropunctus. Phylogenetic analyses confirm that this fungus belongs to the suborder Boletineae (Boletales). This is the first published report of TECM formation in the Boletineae and of sclerotia formation by a Boletus species. Our data suggest that the TECM morphology is an adaptive feature that has evolved separately in two suborders of Boletales (Suillineae and Boletineae) and that TECM formation is controlled by the mycobiont because TECM are found on distantly related angiosperm and gymnosperm host plants.

Key Words: Boletales • ectomycorrhizal fungi • Quercus • rhizomorph • sclerotia • soil ecology • symbiosis • tuberculate ectomycorrhizae

Received for publication 20 February 2009. Accepted for publication 15 May 2009.

FOOTNOTES

¹ The authors thank R. Halling for valuable feedback and assistance in identifying sporocarps, D. Read for information about Boletus porosporus sclerotia, M. Schmull for German translation, M. Binder for sequences of Boletales, M. Frank for laboratory assistance, and E. Smith, F. Smith, and O. Smith for collecting assistance in Massachusetts. G. Bonito, J. Trappe, R. Vilgalys, E. Cazares, G. Guzman, A. Estrada-Torres, J. Garcia, and G. Guevara graciously facilitated collecting by M. E. Smith in Mexico. Funding was provided by the Harvard University Herbaria.

² Author for correspondence (e-mail: mesmith{at}oeb.harvard.edu)

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