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A hierarchical analysis of historical processes and phylogeographic patterns in Salvelinus (Pisces: Salmonidae) Elz, Anna E.

Abstract

Hybrid zones have been studied extensively over the last two decades, but relatively little attention has focused on the historical processes generating hybrid zones. Phylogeography, a subdiscipline of biogeography, evaluates patterns of genetic variation to infer the processes that have shaped the geographic and demographic histories of species and can provide insight into the historical contingencies that facilitated hybridization. Phylogenetic relationships of char (Pisces: Salvelinus) have suggested that hybridization has been a recurring theme throughout their evolutionary history. In this thesis, I used sequence variation in the mitochondrial DNA (mtDNA) control region to perform a hierarchical phylogeographic study that examined intraspecific, interspecific, and trans-species polymorphisms in Salvelinus. I developed a diagnostic assay that identifies bull trout (S. confluentus) mtDNA clades and revealed the presence of both lineages in two coastal watersheds in British Columbia. Headwater stream capture following the last glaciation is suspected to have facilitated this "double invasion". A nested clade analysis (NCA) was performed to evaluate if a widespread hybrid zone between Dolly Varden (S. malma) and bull trout in northwestern North America resulted from two processes: secondary contact of previously isolated lineages and continuous contact resulting from historical introgression in a shared refuge. Despite ambiguity in the statistical parsimony network, NCA provided evidence of secondary contact between Beringian Dolly Varden and bull trout and introgressed (with bull trout mtDNA) Dolly Varden. In addition, I found a degree of phylogeographic substructure amongst bull trout and introgressed Dolly Varden in the Chehalis Refuge, which suggests that introgression has been geographically localized. In a species-level phylogenetic analysis, the "approximately-unbiased test" was used to compare alternate tree topologies and I found that Arctic char lineages were not monophyletic in relation to Beringian Dolly Varden and bull trout. The consensus parsimony tree and the phylogeographic distribution of lineages suggested that both bull trout and Dolly Varden may have experienced historical contact with different Arctic char lineages. I suggest that hybridization has played a significant evolutionary role in the diversification of char. This work presents the most extensive phylogeographic analysis of Salvelinus to date and provides a comparative framework for both small scale and regional studies in northwestern North America.

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