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First published online October 23, 2009; doi:10.3732/ajb.0900101 American Journal of Botany 96: 1997-2009 (2009) © 2009 Botanical Society of America, Inc.

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Mycorrhizal diversity in Apostasia (Orchidaceae) indicates the origin and evolution of orchid mycorrhiza¹

² Tsukuba Botanical Garden, National Museum of Nature and Science, Tsukuba 305-0005 Japan ³ Odum School of Ecology, University of Georgia, Athens, Georgia 30602 USA ⁴ Faculty of Science, Yamagata University, Yamagata 990-8560 Japan

ABSTRACT

We demonstrated that "orchid mycorrhiza," a specialized mycorrhizal type, appeared in the common ancestor of the largest plant family Orchidaceae and that the fungal partner shifted from Glomeromycota to a particular clade of Basidiomycota in association with this character evolution. Several unique mycorrhizal characteristics may have contributed to the diversification of the family. However, the origin of orchid mycorrhiza and the diversity of mycobionts across orchid lineages still remain obscure. In this study, we investigated the mycorrhizae of five Apostasia taxa, members of the earliest-diverging clade of Orchidaceae. The results of molecular identification using nrDNA ITS and LSU regions showed that Apostasia mycorrhizal fungi belong to families Botryobasidiaceae and Ceratobasidiaceae, which fall within the order Cantharellales of Basidiomycota. Most major clades in Orchidaceae also form mycorrhizae with members of Cantharellales, while the sister group and other closely related groups to Orchidaceae (i.e., Asparagales except for orchids and the "commelinid" families) ubiquitously form symbioses with Glomeromycota to form arbuscular mycorrhizae. This pattern of symbiosis indicates that a major shift in fungal partner occurred in the common ancestor of the Orchidaceae.

Key Words: Apostasia • Asparagales • Basidiomycota • Botryobasidiaceae • Cantharellales • Ceratobasidiaceae • evolution • Orchidaceae • mycorrhiza

Received for publication 9 April 2009. Accepted for publication 6 July 2009.

FOOTNOTES

¹ The authors thank K. Fuse, O. Iida, S. Katsuki, S. Kurogi, T. Minamitani, J. Murata, H. Nakayama, M. Saito, T. Saito, K. Sugimura, S. Tagane, M. Takagi, M. Takashima, and N. Tanaka for collecting and H. Abe, T. Fujimoto and T. Yamada for technical assistance. This study is partly supported by Grants-in-Aid to Scientific Research from the Japan Society for Promotion of Science (nos. 17370033, 21370038, and 17255004).

⁵ These authors contributed equally to this work.

⁶ Author for correspondence (e-mail: yukawa{at}kahaku.go.jp)

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