Climatic and geological changes in the Tertiary and Quaternary dramatically influenced the distribution of flora and fauna in North America. In the Tertiary, North America became separated from Eurasia, resulting in vicariant populations of many species of the ancient flora and fauna (Budantsev 1992; Graham 1999). During glacial maxima, plants, fungi and animals were forced to unglaciated refugia, from where they expanded to newly exposed areas in warmer interglacial periods. Reconstruction of these historical events is of paramount importance because they had major influences on past speciation events and are reflected in present day population structures.
Beringia, including Alaska and North-Eastern Siberia, has long been a focal point for biogeographical research in a wide range of plant and animal taxa. This high level of interest arises for two principal reasons. First, due to its diverse landscape and climate and the fact that much of the region remained ice-free during glacial maxima, Beringia served as a refugium for Arctic and subarctic flora and fauna (Adams and Faure 1997; Brubaker et al. 2005; Edwards et al. 2000; Hulten 1968). Second, during much of the Tertiary and the Quaternary periods, Beringia was the major land connection between Asia and North America and provided migration routes to a wide variety of organisms (for example, see Elias et al. 2000; Qian 1999; Swanson 2003). High genetic diversity and evidence for glacial refugia in Beringia have been reported in several, predominantly arctic-alpine animal and plant taxa, such as the Tundra Vole (Microtus oeconomus) (Brunhoff et al. 2003), Bighorn Sheep (Ovis canadensis) and Dall Sheep (O. dalli) (Loehr et al. 2005), Columbian Ground Squirrel (Spermophilus columbianus) (MacNeil and Strobeck 1987), Old World Swallowtail (Papilio machaon) (Sperling and Harrison 1994), Amara alpina, an arctic-alpine ground beetle (Reiss et al. 1999), Paranoplocephala arctica, a parasitic cestode of collared lemmings (Wickström et al. 2003), Entire-leaved Avens (Dryas integrifolia) (Tremblay and Schoen 1999), Purple Saxifrage (Saxifraga oppositifolia) (Abbott and Comes 2003), Arctic Bell-heather (Cassiope tetragona) (Eidesen et al. 2007) and the Bog Blueberry (Vaccinium uliginosum) (Alsos et al. 2005).
Whether fragments of boreal forest existed in Beringia during the Last Glacial Maximum (LGM) is a major, but, as yet, unanswered question in Quaternary science. Although earlier pollen-based biome reconstructions suggest that all of Beringia was covered by arctic tundra with no conifers (Edwards et al. 2000; Kaufman et al. 2004; Swanson 2003), phylogeographic analyses of DNA sequences and the most recent pollen data support the theory that Picea glauca and P. mariana existed in small refugia in Alaska (Anderson et al. 2006; Brubaker et al. 2005).
Ectomycorrhizal (ECM) fungi are obligate root-associated mutualistic symbionts of trees in boreal, temperate and tropical forests (Smith and Read 1997; Bas 1969; Härkönen et al. 1994; Simmons et al. 2002) and their past and present populations likely have been shaped by the distribution of their host trees. The likely importance of host trees in the distribution of ECM fungi has been repeatedly noted.
However, despite their ecological importance, very little is known about ECM phylogeography. Furthermore, learning about the phylogeographic history of ECM taxa not only provides information related to the organism in question, but may also help to resolve whether or not their hosts survived the LGM in Beringian refugia. To test the hypothesis of persistent LGM forest refugia in Alaska, we analysed molecular data from Amanita muscaria, an ECM fungus native to temperate and boreal forest regions of the Northern Hemisphere.
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