Freudenstein, John , Taylor, D. Lee , Barrett, Craig F. .
Coevolution or opportunism? Patterns of association in the mycoheterotrophic orchid Corallorhiza.
In addition to being the largest family of flowering plants, with at least 20,000 species, orchids are the largest clade of angiosperms to be completely dependent on fungi in order to complete their life cycle. While many angiosperms have mycorrhizal relationships with fungi, orchids require a fungus in order to germinate. For the great majority that have been examined (but still a great minority of the whole), these fungal associates belong to the Rhizoctonia anamorphic genus of Basidiomycetes. Most of the leafless, putatively achlorophyllous orchid species have switched to ectomycorrhizal Basidiomycete fungal associates, perhaps because they are able to provide a greater throughput of nutrition. Previous studies of the specificity and taxonomic utilization of fungi indicate that there is often not a one-to-one relationship between orchid species and fungal species. In the leafless mycoheterotroph Corallorhiza we have sampled extensively across the distributions of the species and find a high degree of phylogenetic tracking within most species, such that populations of an orchid species are utilizing associates from a particular fungal clade (Russulaceae or Thelephoraceae). Host shifts occur among Corallorhiza species between distinct groups of fungi, and in at least one case, C. wisteriana, we have detected polymorphism within populations (but not within individuals) for major fungal clades. At the lowest levels, among closely related populations, there is evidence for coevolution. As one takes an increasingly broader view of the genus, host shifts become more prominent, but still will most often be within a well-defined fungal clade. Hence, there is a fundamental level of coevolution and phylogenetic tracking here, with enough flexibility that shifts between major fungal clades are sometimes possible, which could well have implications for evolution in the orchid lineages.
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1 - Ohio State University, Department of Evolution, Ecology, and Organismal Biology, University Herbarium, 1315 Kinnear Rd., Columbus, Ohio, 43212, USA
2 - University of Alaska, Institue of Arctic Biology, 311 Irving I Building, Fairbanks, AK, 99775, USA
3 - Ohio State University, Department of Evolution, Ecology, and Organismal Biology, OSU Herbarium (OS), Museum of Biological Diversity, 1315 Kinnear Rd., Columbus, Ohio, 43212, USA
Presentation Type: Symposium or Colloquium Presentation
Location: Boulevard A/Hilton
Date: Tuesday, July 10th, 2007
Time: 3:00 PM