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Spatial genetic structure in mountain hemlock (Tsuga mertensiana)

University of British Columbia

The spatial distribution of related and unrelated alleles at a geographic, population, or local scale can unravel the relative roles of random genetic drift, mutation, and natural selection in the maintenance of genetic variation. Characterizing spatial and temporal patterns of genetic variation, and the underlying mechanisms, is central to understanding evolutionary and ecological processes like speciation, succession, and the spread and maintenance of a species. Equally important, is knowing how these patterns of variability change with physical scale and how the same processes differentially affect distinct spatial scales. The first part of this study examines mating systems, level of genetic diversity and the distribution of that genetic variation within and among populations of mountain hemlock {Tsuga mertensiana) across British Columbia. The second half of the thesis examines the effects of fragmentation on the demographic structure at the local scale, that is, within a single old growth stand of mountain hemlock. Levels of genetic diversity, inbreeding and isolation by distance are also examined in the natural regeneration surrounding the clearcut. Genetic diversity, mating system and the evolutionary history of 19 populations of mountain hemlock {Tsuga mertensiana) within British Columbia were inferred from genetic variation at 19 allozyme loci. Within populations, 32% of the loci were polymorphic and expected heterozygosity was 0.087 overall populations, which is approximately half the heterozygosity found in other conifers. Outcrossing rates did not significantly differ from 100%. Overall, populations of mountain hemlock across British Columbia showed moderate differentiation (Gst =0.077). Island populations showed considerably more differentation (Gst =0.095) than mainland populations (Gst =0.058), and an isolation-by-distance analysis suggested gene flow was not restricted. For the populations in southwestern British Columbia, there was a significant positive correlation between average expected heterozygosity and elevation, while expected heterozygosity was negatively correlated with latitude. The low genetic diversity suggests that during a northward post-glacial range expansion, more northerly mountain hemlock populations suffered a loss in genetic variation due to this migration. In an old growth mountain hemlock population in southern British Columbia, genetic diversity and relatedness were examined using two microsatellite loci. Levels of inbreeding were found to be significantly different from zero, and increased exponentially to an asymptote as mean diameter increased. High levels of inbreeding may have resulted from family clustering, the presence of null alleles, and spatial or temporal Wahlund effects. The difference in inbreeding levels between trees established before and after fragmentation may be suggestive of a temporal shift in breeding system. Seedlings from the 0-2cm diameter class (postfragmentation) were found to have the highest genetic diversity and lowest levels of inbreeding. Seedlings and adults also differed in their allelic distribution suggesting that seedlings were not simply a subset of the neighbouring adult gene pool. The 1976 clearcut of 43.3 ha surrounding the old growth patch may have substantially altered the density of trees causing a change in wind currents, subsequently enhancing gene flow. Significant relatedness was detected among adult trees 5m apart and in the clearcut between seedlings 2m apart. Both natural regeneration and adult trees exhibited significant isolation by distance when pairwise estimates of relatedness were plotted as a function of increasing pairwise distance. The local genetic structure in mountain hemlock can be attributed to limited seed dispersal, seedling recruitment over a long period of time and long distance founding events.

Thesis/Dissertation

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2009-07-23

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http://hdl.handle.net/2429/11213

Forestry

University of British Columbia

UBCV

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