Submitted to: Molecular Biology and Evolution
Publication Type: Abstract Only
Publication Acceptance Date: 5/8/2009
Publication Date: 7/1/2009
Citation: Bergemann, S.E., Baumgartner, K., Fujiyoshi, P.T., Bruhn, J. 2009. RECONSTRUCTING THE EVOLUTIONARY HISTORY OF THE FOREST FUNGAL PATHOGEN, ARMILLARIA MELLEA, IN A TEMPERATE WORLDWIDE POPULATIONS. Molecular Biology and Evolution. Interpretive Summary:
Technical Abstract: The forest pathogen Armillaria mellea s.s. (Basidiomycota, Physalacriaceae) is among the most significant forest pathogens causing root rot in northern temperate forest trees worldwide. Phylogenetic reconstructions for A. mellea show distinct European, Asian and North American lineages. The North American lineages are further subdivided between eastern and western lineages, although possible dispersal or vicariant patterns among the lineages are obscured by incongruent phylogenetic reconstructions among loci. We are utilizing nuclear microsatellite markers to compare the patterns of genetic variation of A. mellea within North America as well as using variable nuclear and mitochondrial loci (atp6 = mitochondrial atp synthase 6, efa = nuclear elongation factor alpha, gpd = nuclear glyceraldehyde-3-phosphate dehydrogenase) to reconstruct the evolutionary history of A. mellea from worldwide populations. For estimating population structure within North America, we have genotyped a total of 67 genetic individuals from eastern North America and 59 from western North America and reconstructed haplotypes from sequenced regions for a subset of these. In both eastern and western North America, population differentiation across locations indicated no evidence of population structure. Assignment tests using microsatellite allele frequencies in STRUCTURE grouped all isolates of western North America into one cluster. Within eastern North America, five clusters were defined, and individuals across the range of A. mellea (including the midwest, northeast and southeast) were assigned with nearly equal frequency to each cluster. This suggests that there are no limitations to dispersal within eastern or western North America, perhaps as a result of a large, interbreeding population within a broad, contiguous host range. In western North America, we have identified two divergent atp6 haplotypes co-occurring in subpopulations across the geographic range of A. mellea. Coalescent genealogies indicate an ancient split between these two haplotypes and there is little congruence between the geographic origin of mitochondrial or nuclear haplotypes and their position in the haplotype network. Consequently, it does not seem plausible that they diverged in situ. In eastern North America, clusters identified in microsatellite loci are congruent with haplotype diversity of nuclear gpd and efa loci, although the evolutionary and historical processes affecting these patterns are yet, unknown. To explain the differences in patterns observed in eastern North America, we are currently investigating the possibility of: 1) multiple colonization events in eastern North America from different source populations coupled with hybridization events either in ancestral populations or in situ ; 2) maintenance of ancestral polymorphisms from large, ancestral source populations.