Graduate Thesis Or Dissertation

Patterns of natural selection and demography in coastal Oregon coho salmon (Oncorhynchus kisutch) populations : evidence from neutral and olfactory receptor gene-linked markers

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  • For Pacific salmon, the evolution of local adaptations depends upon the species' propensity to return, or "home", to natal streams at time of reproduction. Pacific salmon use olfactory cues to guide homing behavior, yet little is known about the genetics of olfaction in salmon. In this study, I use putatively neutral microsatellite markers to estimate demographic parameters and describe the population genetic structure of Oregon Coastal coho salmon (Oncorhynchus kisutch). Microsatellite analyses indicated weak population structure among coho populations (overall θ = 0.021), modulated by moderate levels of migration (straying). Allelic richness was higher in wild populations than both hatchery populations and wild populations from lake dominated systems. The Coos and Nehalem river populations appeared to be primary migrant sources, possibly elevating allelic richness for central coast populations. I then used genomic sequence data from nine species of salmon and trout to infer the evolutionary history for eight olfactory receptor genes, representing two major gene classes (main olfactory receptors and ORAs). Through a maximum likelihood based analysis of site-specific, non-synonymous to synonymous nucleotide substitution rates, I found strong evidence for positive selection having influenced the diversification of four paralogous main olfactory receptor genes. Main olfactory receptor orthologues appeared highly conserved among species, yet site-specific positive selection may be affecting interspecific divergence of an ORA gene in salmonids. Finally, I used molecular markers linked to olfactory receptor genes to test for a signal of selection among coho salmon populations from different rivers. By examining interlocus variance of F[subscript]ST, I found evidence for directional selection on an olfactory receptor gene-linked marker in coho salmon populations. Pairwise θ values calculated from gene-linked markers were nearly an order of magnitude greater than observed for putatively neutral microsatellites.
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