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Molecular evidence of current and historical introgressive hybridization between bull trout (Salvelinus confluentus) and Dolly Varden (S. malma) Redenbach, Zoë

Abstract

Natural hybridization, once thought to be relatively rare, has been widely observed in both plants and animals. Studies of species that hybridize at low rates across wide ranges, however, are still relatively rare. Dolly Varden (Salvelinus malma) and bull trout (S. confluentus) have widely overlapping ranges and have been shown to hybridize at two localities. This thesis is a molecular analysis of Dolly Varden/bull trout hybridization over a range of spatial scales. Large-scale mtDNA phylogeography revealed that Dolly Varden survived the last glaciation in a previously unsuspected refuge south of the ice sheet, which resulted in continuous sympatry of a Dolly Varden refugial population with bull trout over the last 100,000 years. Discordant mitochondrial and nuclear phylogenies revealed historical introgression of bull trout mitochondrial DNA into Dolly Varden. Population samples revealed widespread local sympatry and current hybridization throughout the overlapping range, with hybrids consisting of 0 to 25% of the local samples. A detailed analysis of hybridization within a single watershed in north-central British Columbia revealed that individuals of hybrid origin form 9% of the juvenile population of char (0.5% F₁, 0.5% Fn, and 8% backcross genotypes). Initial interspecific hybridization is unidirectional, Dolly Varden male by bull trout female, likely attributable to a 'sneaking' mating strategy used by smaller Dolly Varden males. The F₁ hybrids were fertile and backcrossed with both parental species. The introgression of nuclear and mitochondrial alleles was asymmetrical, with bull trout mtDNA and Growth Hormone 2 introgressing into Dolly Varden at higher levels than both the three other nuclear loci and the reverse direction introgression of Dolly Varden alleles into bull trout, suggesting that the strength of selection can vary across nuclear loci and between species. As Dolly Varden and bull trout appear to be distinct species in sympatry, despite introgressive hybridization, a balance between gene flow and selection must be in place. I suggest that selection acts primarily against the adult hybrid population, as the morphological and behavioural intermediacy of hybrid and backcross individuals as adults would affect their potential reproductive success and their ability to succeed in either of the two alternate life histories bull trout and Dolly Varden adopt in sympatry (adfluvial vs. stream-resident).

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