Population structure suggests reproductively isolated lineages of Phaeocryptopus gaeumannii

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Abstract
A survey of the genetic diversity and population structure of the Douglas-fir Swiss needle cast pathogen Phaeocryptopus gaeumannii was conducted with single-strand conformational polymorphisms (SSCP) to screen for variability in mitochondrial and nuclear housekeeping genes. Thirty host populations representing the natural range of Douglas-fir as well as locations where the tree was planted as an exotic were sampled. Sequencing of SSCP variants revealed that the method accurately detected both single nucleotide and indel polymorphisms. Sequence information was used to construct multilocus gene genealogies and to test various hypotheses of recombination (outcrossing) and clonality (selfing). We found that P. gaeumannii in the region of Oregon’s Swiss needle cast epidemic exhibits strong multilocus gametic phase disequilibrium and is subdivided into two reproductively isolated sympatric lineages. Low genotypic diversity together with the presence of overrepresented genotypes in both lineages suggests a predominantly selfing reproductive mode. Genotypes of one lineage were found in isolates from a widespread geographic distribution, occurring throughout much of the Pacific Northwest as well as nonindigenous populations abroad that have historical reports of disease. Genotypes of the second lineage were detected only in isolates from Oregon’s coastal region. Within the main epidemic area, abundance of this second lineage in young plantations appeared to be correlated with disease severity.