POPULATION STRUCTURE SUGGESTS REPRODUCTIVELY ISOLATED LINEAGES OF PHAEOCRYPTOPUS GAEUMANNII.

Authors: WINTON, LORETTA; HANSEN, E - OREGON STATE UNV.; STONE, J - OREGON STATE UNV.

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2006
Publication Date: 9/20/2006
Citation: Winton, L.M., Hansen, E.M., Stone, J.K. 2006. Population structure suggests reproductively isolated lineages of Phaeocryptopus gaeumannii. Mycologia 98(5):781-791.

Interpretive Summary: Swiss needle cast has historically caused economically significant disease only where Douglas-fir has been planted as an exotic species or Christmas tree crop. Until recently this disease was considered a classic example of a normally benign plant parasite becoming pathogenic where its host is grown beyond its native range. The causal agent of the disease, Phaeocryptopus gaeumannii, is believed to be endemic to the Pacific Northwest and is ubiquitous on Douglas-fir throughout its native region in western North America. Recently Swiss needle cast has been implicated in a serious decline of Douglas-fir along the coastal region of Oregon. We used DNA sequence variation to investigate the population biology of P. gaeumannii from a worldwide collection of isolates in order to evaluate the hypothesis that either an aggressive European race has been introduced to Oregon, or a new mutation has arisen locally. We found that P. gaeumannii in the region of Oregon's Swiss needle cast epidemic is subdivided into two reproductively isolated lineages, or cryptic species. One lineage was widely distributed in the Pacific Northwest as well as in locations abroad with historical reports of disease. The second lineage was restricted to western Oregon and suggested a correlation with symptoms of Swiss needle cast. These results are important to plant pathologists and forest and nursery managers attempting to control the disease, as well as to mycologists interested in fungal population biology in natural and managed systems.

Technical Abstract: A survey of the genetic diversity and population structure of the Douglas-fir Swiss needle cast pathogen Phaeocryptopus gaeumannii was conducted using 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, so sequence information at informative sites 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 non-indigenous 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.