Mitochondrial DNA variation in bull trout (Salvelinus confluentus) from northwestern North America: implications for zoogeography and conservation

Bull trout, Salvelinus confluentus (Salmonidae), are distributed in northwe stern North America from Nevada to Yukon Territory, largely in interior dra inages. The species is of conservation concern owing to declines in abundan ce, particularly in southern portions of its range. To investigate phylogen etic structure within bull trout that might form the basis for the delineat ion of major conservation units, we conducted a mitochondrial DNA (mtDNA) s urvey in bull trout from throughout its range. Restriction fragment length polymorphism (RFLP) analysis of four segments of the mtDNA genome with 11 r estriction enzymes resolved 21 composite haplotypes that differed by an ave rage of 0.5% in sequence. One group of haplotypes predominated in 'coastal' areas (west of the coastal mountain ranges) while another predominated in 'interior' regions (east of the coastal mountains). The two putative lineag es differed by 0.8% in sequence and were also resolved by sequencing a port ion of the ND1 gene in a representative of each RFLP haplotype. Significant variation existed within individual sample sites (12% of total variation) and among sites within major geographical regions (33%), but most variation (55%) was associated with differences between coastal and interior regions . We concluded that: (i) bull trout are subdivided into coastal and interio r lineages; (ii) this subdivision reflects recent historical isolation in t wo refugia south of the Cordilleran ice sheet during the Pleistocene: the C hehalis and Columbia refugia; and (iii) most of the molecular variation res ides at the interpopulation and infer-region levels. Conservation efforts, therefore, should focus on maintaining as many populations as possible acro ss as many geographical regions as possible within both coastal and interio r lineages.