Population structure of Russula brevipes across stands, hosts, and geographic regions

Little research has been conducted specifically on mechanisms that maintain ectomycorrhizal populations or the dynamics of genetic variation in natural forest situations. The objective of this research was to assess the factors influencing the genetic structure of Russula brevipes, one of the most widely dispersed and abundant late successional fungi found in conifer forests, at local, population and biogeographic scales. Allelic variation in microsatellite loci and mitochondrial DNA polymorphisms were analyzed in several populations to determine population structure across stands and hosts, and to gain insight into mechanisms of genetic differentiation such as dispersal, degree to which populations were isolated from other populations, and historical factors that shaped population structure. To assess population structure, sporocarps were collected in Douglas-fir, lodgepole pine, and Sitka spruce stands in regions of the Pacific Northwest and Rocky Mountains. Allelic variation in microsatellite loci indicated no evidence of host genetic structuring in populations of R. brevipes. In the Pacific Northwest, genetic structuring indicated a divide across Oregon separating populations collected in Washington and northern Oregon from populations in northern California and central Oregon. Significant regional population structuring suggested that mechanisms of spore dispersal in R. brevipes are not effective for long distance transport. In addition, identification of two populations with distinct genetic profiles with little overlap in allele frequencies suggested that barriers to mating likely gave rise to isolated populations in Sitka spruce. Genotypic diversity was measured by directly estimating the number of clones from genetic markers, mapping genotypes in local populations, and estimating the extent of migration and recombination in subpopulations of R. brevipes. The results of the genetic diversity analyses indicated that populations were structured in geographic regions with infrequent migration and mating in localized populations in the Pacific Northwest. In the Rocky Mountains, populations consisted of low genotypic diversity resulting from genetically distinct source populations. Populations of R. brevipes are genetically structured in regional populations with little gene flow across broad geographic scales.