Divergence in mycorrhizal specialization within Hexalectris spicata (Orchidaceae), a nonphotosynthetic desert orchid.

Evidence is accumulating for specialized yet evolutionarily dynamic associations between orchids and their mycorrhizal fungi. However, the frequency of tight mycorrhizal specificity and the phylogenetic scale of changes in specificity within the Orchidaceae are presently unknown. We used microscopic observations and PCR-based methods to address these questions in three taxa of nonphotosynthetic orchids within the Hexalectris spicata complex. Fungal ITS RFLP analysis and sequences of the ITS and nuclear LSU ribosomal gene fragments allowed us to identify the fungi colonizing 25 individuals and 50 roots. Thanatephorus ochraceus(Ceratobasidiaceae) was an occasional colonizer of mycorrhizal roots and nonmycorrhizal rhizomes. Members of the Sebacinaceae were the primary mycorrhizal fungi in every Hexalectris root and were phylogenetically intermixed with ectomycorrhizal taxa. These associates fell into six ITS RFLP types labeled B through G. Types B, C, D, and G were found in samples of H. spicata var. spicata, while only type E was found in H. spicata var. arizonica and only type F was found in H. revoluta. These results provide preliminary evidence for divergence in mycorrhizal specificity between these two closely related orchid taxa. We hypothesize that mycorrhizal interactions have contributed to the evolutionary diversification of the Orchidaceae. Interactions between species, especially intimate symbioses, have played major roles in the adaptive evolution and diversification of life. In plants, interactions with herbivores, pathogens, pollinators, seed dispersers, and mutualistic soil microorganisms have been particularly influential. Mycorrhizal symbioses with soil fungi are ancient, possibly dating to the emergence of plants on land (Heckman et al., 2001) and are still critical to the mineral nutrition of most wild plants (Smith and Read, 1997). While mycorrhizal interactions have certainly influenced the evolution of nutrient uptake strategies in plants, most plants do not interact specifically with narrow taxonomic groups of fungi (Molina et al., 1992), making pairwise coevolution between plant and fungal species unlikely (Taylor, 2000). In stark contrast, some members of the species-rich Orchidaceae display marked mycorrhizal specificity toward particular taxonomic groups of basidiomycete fungi (reviewed in Taylor et al., 2002). We use ‘‘specificity’’ to refer to the relative phylogenetic diversity of fungal associates that interact with a particular plant (Thompson, 1994; Taylor and Bruns, 1999b). The mycorrhizal specificity in orchids suggests that mycorrhizal interactions, like pollination syndromes, may have played a role in the diversification of the Orchidaceae. However, we do not know how common mycorrhizal specialization 1