Systematics of the ectomycorrhizal genus Lactariusin the Rocky Mountain alpine zone

Lactarius(Russulales) is an important component of ectomycorrhizal fungal communities in cold-dominated contiguous arctic and disjunct alpine habitats where it associates primarily with Betula, Dryasand Salix. However, little is known of this genus in the central and southern Rocky Mountain alpine zone (3000–3900 m) of North America. Molecular phylogenetic analyses of nuc rDNA ITS1-5.8S-ITS2 (ITS barcode) and the second largest subunit of the RNA polymerase II gene (RPB2) partial sequences in conjunction with detailed morphological examination confirm at least six species occurring above treeline. Most have intercontinental distributions in North America and Eurasia according to molecular comparison with type material and collections from Europe, Fennoscandia, Svalbard and Alaska. Rocky Mountain collections of L. lanceolatus(subgenus Russularia), along with the type from Alaska are paraphyletic with respect to L. aurantiacusand North American taxa L. luculentusand L. luculentusv. laetus. Rocky Mountain collections of L. nanus, L. glyciosmus, L. repraesentaneusand L. salicis-reticulatae(subgenus Piperites) all form clades with European material from type localities and other arctic-alpine habitats. The arctic-alpine L. pseudouvidus/L. brunneoviolaceusgroup appears to be a complex containing additional taxa. North American material originally described as part of this group is well-separated phylogenetically and is described here as L. pallidomarginatussp. nov. Lactarius lanceolatus, L. nanusand L. salicis-reticulataeappear largely restricted to arctic-alpine habitats with Salix. Lactarius glyciosmusand L. repraesentaneusoccur in arctic-alpine, subalpine and boreal habitats with Betulaand also Piceaand possibly Salixfor the latter. Species distributions are hypothesized to be shaped by host ranges, glaciation and long distance dispersal.