Although the profound impacts of Pleistocene glaciations on the North American aquatic fauna are indisputable, reconstructions of postglacial recolonization are extremely difficult due to the complexity of habitat alterations and glacial retreat. This study examines patterns of mitochondrial DNA (mtDNA) diversity in lake trout (Salvelinus namaycush) and arctic charr (Salvelinus alpinus) to assess their phylogeographic structure and postglacial dispersal. Phylogeographic results for both species contrasted with literature predictions, but were concordant with dispersal scenarios based on glaciochronology and species ecology.

Restriction fragment length polymorphism (RFLP) analysis of mtDNA revealed five distinct refugial groups of S. namaycush that diverged 100,000-800,000 years ago. Screening of 123 populations showed that fish from two Beringian refugia colonized much of arctic and western Canada and dispersed as far east as western Quebec. Mississippian fish dispersed throughout central Canada, while eastern North America was recolonized from an Atlantic source. Fish from a fifth refuge are largely limited to Montana and southern Alberta. Refugial groups showed extensive secondary contact within margins of former proglacial lakes, producing high levels of nucleotide diversity for mixed-source populations. Geographic distance and genetic divergence among refugial groups were not correlated, contrasting with patterns for southern fish species.

RFLP analysis of North American S. alpinus mtDNA revealed three major lineages with differing geographic distributions. Screening of 54 populations showed that two lineages that diverged 120,000-250,000 years ago in separate Atlantic refugia showed clinal distributions along the Atlantic coast. By contrast, northern Alaska and arctic Canada were colonized by Beringian fish, which diverged from eastern populations one to two million years ago. In contrast to lake trout, secondary contact among arctic charr lineages was extremely limited.

Combined morphometric, allozyme, and RFLP analyses also documented widespread hybridization between S. namaycush and S. alpinus across the Canadian arctic. Hybridization was strongly asymmetrical, with 87% of $\rm F\sb1$ hybrids resulting from crosses between female S. namaycush and male S. alpinus. Detailed examination of two populations containing hybrids revealed low levels of bidirectional nuclear introgression between the species but only limited transfer of S. namaycush mtDNA into S. alpinus.