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Beautiful but lacking diversity : population genetics of Pacific Dogwood (Cornus nuttallii Audobon ex Torr. & A. Gray) Keir, Karolyn R.


In the past, conifers have been the primary focus of population and conservation genetic studies in Pacific Northwest (PNW) trees. These studies have provided tremendous insight as to how genetic diversity varies across species ranges for these wind-pollinated and mostly wind-dispersed species. With this study of Pacific dogwood (Cornus nuttallii), a broadleaved, PNW species, which utilizes biological vectors for pollen and seed dispersal, we hope to broaden our understanding of tree evolutionary dynamics. Marker development for C. nuttallii found few useful polymorphisms. Of eight microsatellite markers (SSRs) developed from a closely related species, three were monomorphic, while the other five averaged only 4.4 alleles/locus. Furthermore, only a single base pair substitution was found in the rpl16 region of the chloroplast genome after sequencing 2,262 non-coding base pairs in 100 individuals. This lack of diversity, which was found to be ubiquitous throughout the range of C. nuttallii, suggests this species may have endured a prolonged bottleneck in a single glacial refugium prior to recolonization. The cpDNA phylogeographic pattern and a significant decline in both SSR allelic richness (r² = 0.42, p<0.01), and expected heterozygosity (r² = 0.51, p<0.01) support this theory. Low levels of population structure, documented in both chloroplast (D = 0.153) and nuclear genomes (FST = 0.071, RST = 0.036) may suggest high levels of contemporary gene flow between populations are also influencing current patterns of diversity. Despite variation being the precursor for adaptation, a comparison of QST (0.088 for first-year height and 0.113 for bud burst timing) with a refined FST estimate (0.053), indicated that C. nuttallii had either retained or recovered significant phenotypic variation for differential selection to act. Such uniformly low diversity raises the issue of how genetic conservation efforts should proceed with this and other species sharing a similar degree of genetic depauperateness. So that signs of decline may be detected, we suggest population monitoring, especially for those populations occurring at high elevations. Furthermore, we advocate the transfer of seeds from the nearest southern source, in the event that restorative efforts are required to assist this species to cope with the rapidly changing climate.

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