The fungus Nothophaeocryptopus gaeumannii is the causative agent of Swiss needle cast (SNC), a foliar disease of Douglas-fir. Disease is characterized by premature loss of foliage and reduced growth resulting from the inhibition of photosynthesis due to the occlusion of stomata by the ascocarps of N. gaeumannii. Although the disease was first reported from Douglas-fir plantings in Europe, the fungus that causes the disease is endemic to the native range of Douglas-fir in western North America. The focus of this dissertation is to evaluate the factors that influence the genetic structure of native and introduced populations of N. gaeumannii and determine whether population structure is related to SNC severity.
Native populations of this fungus in western North America are subdivided into two lineages that may constitute cryptic species. Evidence suggests that one of these lineages (Lineage 2) may be more aggressive, as it appears to be more abundant relative to Lineage 1 where SNC is most severe. We determined that the two lineages are highly differentiated and do not appear to interbreed but are sympatric at very fine spatial scales. In addition to this population subdivision, homothallic reproduction, inferred based on the presence of repeated multilocus genotypes (MLGs), was found to be a major factor influencing the genetic structure of N. gaeumannii in native and introduced populations worldwide. Spatial genetic variation in these populations allowed us to infer gene flow and migration at various hierarchical scales. For instance, in New Zealand, human-mediated migration of infected trees between sites seemed to be the most parsimonious explanation for the observed spatial distributions of repeated MLGs, as some MLGs were shared at sites over 1,000 km apart. This suggests that the movement of infected Douglas-fir seedlings by the timber or ornamental nursery trades may be a significant factor influencing the contemporary structure of this introduced population.
In Chapter four a dataset consisting of MLGs from 3,829 N. gaeumannii isolates from North America, South America, Europe, New Zealand, and Australia was analyzed to assess diversity and genetic structure in the global N. gaeumannii population. Approximate Bayesian computation (ABC) allowed for the identification of the most probable introduction pathways. This computational tool was used to test various demographic scenarios, each of which represented hypotheses about the origins of the introduced populations. This analytical framework also allowed for an assessment of the influences of founding events and population bottlenecks on the genetic structures of introduced populations. The demographic scenarios with the highest statistical support suggested that N. gaeumannii was first introduced from North America to Europe, and more recently from North America to New Zealand and South America. The N. gaeumannii population in Australia was likely introduced from Europe, and thus the populations in Australia and New Zealand were introduced from different sources despite the relative proximity of the two countries.
The results presented in Chapter five suggested that environment is also an important factor shaping the genetic structure of native N. gaeumannii populations. The same environmental variables that were associated with SNC severity were also correlated with genetic variation in the pathogen population. For instance, the geographic distribution of Lineage 2 had a strong positive correlation with mean winter temperature. At both the landscape level and the stand level, it appeared that the spatial distribution of Lineage 2 was correlated with SNC severity. However, after accounting for the environmental differences between each of the sites associated with their relative coastal proximity, there was no evidence to suggest a relationship between the relative proportion of Lineage 2 and SNC severity. This suggests that the superficial association between N. gaeumannii Lineage 2 and disease severity is likely a reflection of the environmental influences on the disease and the differential environmental preferences of the two pathogen lineages. There was also strong genetic differentiation between the N. gaeumannii isolates collected from sites with the highest SNC severity and those collected from the sites with the lowest SNC severity, suggesting that there may have been genetic variation associated with disease or environment independent of the relative abundances of the two lineages. The evidence presented here suggested that climate is an important determinant of SNC severity, but also that climate has influenced the structure of N. gaeumannii populations.