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Genetic diversity and spatial population structure of Sitka spruce (Picea sitchensis (Bong.) Carr.) : implications for gene conservation of widespread species Gapare, Washington Jingo

Abstract

Knowledge of genetic diversity and population structure is of fundamental importance in the development of gene conservation sampling strategies to capture and preserve allelic diversity. Such knowledge is critical if we are to understand how to manage and maintain diversity in species and populations. To assess the effects of sampling strategy on capture of allelic diversity in widespread species, I studied Sitka spruce (Picea sitchensis (Bong.) Carr) populations as a model. Sitka spruce is a conifer that occupies wide geographic and ecological niches from 33° N to 60° N latitude along the Pacific coast of North America. A total of 1600 individual trees were sampled in eight populations classified as core or peripheral based on ecological niche, and continuous or disjunct based on distribution. In each population, 200 trees were spatially mapped and genotyped for eight cDNA-based sequence-tagged-site (STS) co-dominant markers. One important finding of this study is the similarity in genetic diversity as measured by expected heterozygosity between core populations (mean HE = 0.58) and peripheral populations (mean HE = 0.56). Another remarkable result found by this study is strong spatial structure as evidenced by coancestry in peripheral populations, both continuous and disjunct, but not in core populations. For example, trees located within 50 metres of each other in peripheral, disjunct populations had coancestry values greater than 0.20 while in core populations, trees within the same distance class had coancestry values below 0.06. Differences in population structure were attributed to an aggregation of similar multi-locus genotypes, in a structured, isolation by distance manner in peripheral populations, both continuous and peripheral but not in core, continuous populations. Irrespective of population classification, over 75% of the alleles were common and widespread. Only one allele, representing two percent of all alleles was classified as rare and localized on average, and this allele was limited to one core, disjunct and two peripheral, disjunct populations. To capture localized alleles (both common and rare), sampling should cover more populations over the geographic and ecological range of species at a cost of fewer individuals per population. The conservation of peripheral populations may present the best opportunity for preserving rare alleles.

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