The genetic impact of natural and human induced historical events on eastern white pine (Pinus strobus) assessed using nuclear and organelle genomic markers
University of New Brunswick
Eastern white pine (Pinus strobus) is an important conifer species that plays an integral role in the natural landscape and human infrastructure of eastern North America. In order to understand the current population-genetic structure of this species and factors influencing its distribution, I used neutral microsatellites and adaptive single nucleotide polymorphisms (SNPs) markers to examine the population-genetic structure across the range and smaller-scale structure between core and marginal populations, and continuous vs. fragmented populations. Across the species range I observed low but significant levels of genetic differentiation and significant genetic structure. I observed broad consensus from multiple markers (nuclear and chloroplast microsatellites and mitochondrial SNPs) supporting the presence of two main post glacial lineages, east of the Appalachian mountains and south to west of the Great Lakes, that originated from a single southern refuge in the mid-Atlantic plain. Substructure was observed in the eastern lineage, subdividing it into coastal and central groups. These findings showed for the first time in a widely distributed, wind pollenated conifer the presence of multiple colonization routes in eastern North America and the presence of at least three Evolutionarily Significant Units. Populations in the geographical core of a species' range are expected to show higher genetic diversity and lower genetic differentiation than populations at the margin, as a result of lower effective population size, higher genetic drift, increased fragmentation, genetic isolation and increased selective pressures in the margin of the range. I observed none of this degradation in eastern white pine when comparing the genetic characteristics of 11 core and 11 marginal populations throughout the species' range. Genetic diversity and differentiation did not differ significantly between core and marginal populations, suggesting that despite landscape fragmentation and extensive harvesting, marginal eastern white pine populations can maintain genetic diversity and connectivity. This may be due to eastern white pine's predominantly outcrossing mating system, inbreeding depression and long-distance gene flow. As a result of substantial human induced decreases in population size and increased physical fragmentation heavily impacted forests are expected to show lower genetic diversity and higher genetic differentiation. I compared the genetic characteristics of populations in continuous forests and fragmented forests, from southern New Brunswick, Canada, to determine how landscape fragmentation influenced genetic diversity and structure. Pine in fragmented populations had significantly lower F ls (a measure of inbreeding), but I found no significant difference between population types for other measures of genetic diversity and population genetic structure. Despite extensive harvesting over the past 300 years, second growth pine populations have maintained genetic diversity and connectivity at levels similar to undisturbed old growth stands. Together, my findings suggest that the genetic diversity and population structure of eastern white pine populations has remained relatively unchanged since the arrival of European settlers. The current genetic distribution is the result of historical (phylogeographic) and presumptively adaptive response to climate variation, not fragmentation or population disruption due to human activity.