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Genomic and phenotypic architecture of a spruce hybrid zone (Picea sitchensis x P. glauca) Hamilton, Jill Adelle


Natural hybrid zones may be viewed as important biological systems for examining the role of selection in creating and maintaining species differences. Where ecological differences exist between hybridizing species, these zones may provide useful insight into the genetic architecture of important traits involved in adaptation. I have evaluated the genomic and phenotypic architecture of the economically and ecologically important Picea sitchensis (Sitka spruce) x P. glauca (white spruce) hybrid zone along the Nass and Skeena river valleys in northwestern British Columbia using chloroplast and mitochondrial markers, twelve microsatellite loci (SSRs), and 268 single nucleotide polymorphisms (SNPs), in combination with morphological variation, and phenotypic data from a common garden. Maternally- and paternally-inherited organelle markers, in combination with bi-parentally inherited nuclear markers, were used to estimate both the historic and contemporary direction and extent of gene flow within the hybrid zone. Sitka spruce mitotype ‘capture’ throughout the introgression zone point towards asymmetric gene flow, congruent with microsatellites and SNPs, indicating extensive long-term introgression and widespread recombination with more Sitka spruce than white spruce ancestry in hybrid populations. Significant clinal variation was observed for marker-based hybrid indices and morphological traits associated with climate and geography, while growth and cold hardiness traits evaluated in a common garden exhibited weak to non-significant clines. These results indicate extrinsic selection appears to play a strong role in the distribution and structure of this hybrid zone, which fits expectations for the environmentally-determined bounded hybrid superiority model of hybrid zone maintenance. However, intrinsic mechanisms of hybrid zone maintenance could not be ruled out. Finally, broad-scale patterns of variation, combined with fine-scale analysis of candidate SNP-specific patterns of introgression revealed a suite of candidate loci that may be targets of extrinsic or intrinsic selection. These loci may be involved in either adaptation to climate across the zone, particularly precipitation gradients, or involved in the maintenance of species barriers. These results have important implications for genetic conservation of adaptive variation, selection of seed sources for current reforestation within this ecologically transitional area, and appropriate scale and direction of seed transfer relating current genotype-climate associations to future climate predictions for this region.

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