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Transcriptome evolution in black cottonwood (Populus trichocarpa) Ono, Jasmine


In 1975, King and Wilson proposed that gene expression variation can play a role in the evolution of phenotypic variation since the variation in nature could not be explained by variation in protein coding sequences alone. When a mutation which causes a change in gene expression is introduced in to a population, either selection or neutral drift can act on it. When this mutation causes no change in fitness of the organism, it will be affected by neutral drift. The bounds for neutral drift are thought to be set by stabilizing selection. If the mutation is beneficial to the organism, it will be affected by positive selection. When different populations are located in different environmental conditions, different mutations can be beneficial in each population and divergent selection can result. We looked for these patterns of gene expression evolution among populations of Populus trichocarpa, black cottonwood,using a PST vs. FST approach. P. trichocarpa is a model tree system that allows the study of an extended suite of tree biological processes. A suite of genomic tools have been developed for black cottonwood, including a genome sequence and a 15.5K microarray. It is broadly distributed in the far west of North America and shows an ecotypic mode of genetic differentiation, with populations divided into northern and southern groups. In this study, we examined gene expression from 12 P. trichocarpa populations, 6 from the north and 6 from the south. We found evidence for divergent selection acting on the expression values of many genes, as well as stabilizing selection acting on a few. This supports the prevalence of natural selection acting on phenotypic traits, but we still found an overwhelming majority of traits which seem to be drifting neutrally. We found no evidence for different selection acting on the northern and southern groups.

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