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Abstract
The fungal symbionts of arbuscular mycorrhiza form a monophyletic group in the true Fungi, the phylum Glomeromycota. Fewer than 200 described species currently are included in this group. The only member of this clade known to form a different type of symbiosis is Geosiphon pyriformis, which associates with cyanobacteria. Because none of these fungi has been cultivated without their plant hosts or cyanobacterial partners, progress in obtaining multigene phylogenies has been slow and the nuclear-encoded ribosomal RNA genes have remained the only widely accessible molecular markers. rDNA phylogenies have revealed considerable poly-phyly of some glomeromycotan genera that has been used to reassess taxonomic concepts. Environmental studies using phylogenetic methods for molecular identification have recovered an amazing diversity of unknown phylotypes, suggesting considerable cryptic species diversity. Protein gene sequences that have become available recently have challenged the rDNA-supported sister group relationship of the Glomeromycota with Asco/Basidiomycota. However the number of taxa analyzed with these new markers is still too small to provide a comprehensive picture of intraphylum relationships. We use nuclear-encoded rDNA and rpb1 protein gene sequences to reassess the phylogeny of the Glomeromycota and discuss possible implications.
The authors thank Tim James and Arthur Schüßler for providing unpublished sequences from the AFTOL project, Fritz Oehl and Kerstin Wex for providing images, the Botanical Institute at the University of Basel for generous support, Zuzana Sykorova for critically reading the manuscript and the Swiss National Science Foundation for financial support through grants 3100 066688.01 and 3100 A0109466.1 to D.R. We also acknowledge support from NSF 0090301, Research Coordination Network: a phylogeny for kingdom Fungi to M. Blackwell, J.W. Spatafora and J.W. Taylor.
