UBC Theses and Dissertations
Genomics of western redcedar (Thuja plicata) Shalev, Tal James
Western redcedar (WRC; Thuja plicata) is an ecologically, culturally, and economically important tree species. It is a key successional tree species and a major component of old-growth stands in its range, has been revered and used by First Nations of the Pacific Northwest of North America for millennia, and represents a $1.1B industry. WRC is unique among conifers in that it readily self-fertilizes (selfs) and does not appear to be affected by inbreeding depression for most fitness traits. Further, its tolerance of environmental stressors makes it a focal tree species for future range expansion due to climate change. Development of genomic resources and methods for conifers and other trees has become fundamental over the last decade for better understanding their biology and expediting tree improvement programs for growth and defense traits. As operational forestry shifts from old-growth to second-growth harvesting, using genomic data to better understand the population genetics, demography, and important growth and defense traits of WRC is essential. In this thesis I present novel genomic resources for WRC. These are then used to explore genetic diversity, mating system, and terpene chemistry traits associated with defense against ungulate browsing and fungal pathogens. I found that the genetic diversity of WRC is exceptionally low for a continuous tree species of its range size, likely due to range expansion from a single glacial refugium and its unique self-compatibility. Analysis of selfing lines revealed that WRC’s adaptability and responsiveness to selection may be due to balancing selection as a result of its demography. Further, not only does WRC show limited inbreeding depression for growth, terpene chemistry, or dendrochronological traits, but appropriate selection intensity can mitigate the effects of inbreeding for growth traits. I found that terpene synthase (TPS) genes in WRC occupy a unique clade from other conifers, and that many loci were associated with terpene chemistry traits in WRC, with substantial variation in effect sizes of loci between different traits. This work serves as a foundation for future genomic research, and provides fundamental data for improving breeding prospects for traits of interest in WRC.
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