Arbuscular mycorrhizal associations in mycoheterotrophic ferns and lycopods

Most land plants (from liverworts to angiosperms) form mutualistic arbuscular mycorrhizal (AM) symbioses with fungi in the Glomeromycota (glomalean fungi) where the plant host gains essential mineral nutrients from the fungal symbiont in exchange for fixed carbon. However, current understanding of the diversity, physiology, ecology, and evolution of glomalean fungi symbioses is mostly limited to AM associations in photosynthetic angiosperms. Notably, most studies have overlooked non-photosynthetic plants that have mycoheterotrophic AM associations where the plant host most likely gains all organic carbon through their glomalean symbionts.;I employed a molecular phylogenetic approach to answer fundamental questions concerning the phylogenetic identity, distribution, ecology and evolution of mycoheterotrophic AM symbionts throughout the life cycle of ferns ( Botrychium and Psilotum) and lycopods ( Huperzia and Lycopodium). These lineages provided a unique opportunity to study AM associations within the context of a widespread type of life cycle involving a mycoheterotrophic gametophyte and a photosynthetic sporophyte.;My sequence data and phylogenetic analyses provide an emerging picture of the ecological and evolutionary relationships between plants with mycoheterotrophic life cycle phases and their fungal partners. The distribution of AM symbionts between the different life cycle phases and neighboring photosynthetic plants provide circumstantial evidence that mycoheterotrophic phases are gaining fixed carbon from neighboring conspecific and/or heterospecific photosynthetic sporophytes through shared fungal networks.;My phylogenetic analyses demonstrate that most AM mycoheterotrophic plants form associations with narrow lineages of Glomus A. Glomus A is the largest and most diverse fungal group known to form AM associations with most land plants. Given that the fungal symbionts recovered from mycoheterotrophic ferns, lycopods and angiosperms encompass the known diversity of Glomus A, my data suggest that all members of Glomus A may ultimately be found to form mycoheterotrophic AM associations. If true, Glomus A composes a fungal clade that has the ability to form complex labile AM networks where the fungus may receive or transfer carbon to the plant host. Thus, my research suggests a new framework for studying the ecology and evolution of "mutualistic" AM associations where carbon flow between separate plants in a community is potentially widespread through shared Glomus A networks.