title: Genetic and phenotypic variation along an ecological gradient in lake trout Salvelinus namaycush.
abstract: Adaptive radiation involving a colonizing phenotype that rapidly evolves into at least one other ecological variant, or ecotype, has been observed in a variety of freshwater fishes in post-glacial environments. However, few studies consider how phenotypic traits vary with regard to neutral genetic partitioning along ecological gradients. Here, we present the first detailed investigation of lake trout Salvelinus namaycush that considers variation as a cline rather than discriminatory among ecotypes. Genetic and phenotypic traits organized along common ecological gradients of water depth and geographic distance provide important insights into diversification processes in a lake with high levels of human disturbance from over-fishing.
  
  
  
  Four putative lake trout ecotypes could not be distinguished using population genetic methods, despite morphological differences. Neutral genetic partitioning in lake trout was stronger along a gradient of water depth, than by locality or ecotype. Contemporary genetic migration patterns were consistent with isolation-by-depth. Historical gene flow patterns indicated colonization from shallow to deep water. Comparison of phenotypic (Pst) and neutral genetic variation (Fst) revealed that morphological traits related to swimming performance (e.g., buoyancy, pelvic fin length) departed more strongly from neutral expectations along a depth gradient than craniofacial feeding traits. Elevated phenotypic variance with increasing water depth in pelvic fin length indicated possible ongoing character release and diversification. Finally, differences in early growth rate and asymptotic fish length across depth strata may be associated with limiting factors attributable to cold deep-water environments.
  
  
  
  We provide evidence of reductions in gene flow and divergent natural selection associated with water depth in Lake Superior. Such information is relevant for documenting intraspecific biodiversity in the largest freshwater lake in the world for a species that recently lost considerable genetic diversity and is now in recovery. Unknown is whether observed patterns are a result of an early stage of incipient speciation, gene flow-selection equilibrium, or reverse speciation causing formerly divergent ecotypes to collapse into a single gene pool.
authors.affiliation: Marine Gene Probe Lab, Department of Biology, Dalhousie University, 1355 Oxford Street, PO Box 15000, Halifax, NS, B3H 4R2, Canada. s.m.baillie@gmail.com.
authors.forename: Shauna M
authors.initials: SM
authors.lastname: Baillie
authors.affiliation: Great Lakes Fishery Commission, 2100 Commonwealth Boulevard, Ann Arbor, MI, 48105, USA.
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Muir
authors.affiliation: U.S. Geological Survey, Great Lakes Science Center, Hammond Bay Biological Station, 11188 Ray Road, Millersburg, MI, 49759, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Hansen
authors.affiliation: Department of Fisheries and Wildlife, Center for Systems Integration and Sustainability, Michigan State University, East Lansing, MI, 48824-1222, USA.
authors.forename: Charles C
authors.initials: CC
authors.lastname: Krueger
authors.affiliation: Marine Gene Probe Lab, Department of Biology, Dalhousie University, 1355 Oxford Street, PO Box 15000, Halifax, NS, B3H 4R2, Canada.
authors.forename: Paul
authors.initials: P
authors.lastname: Bentzen
mesh_terms: D000818:Animals; D017753:Ecosystem; D051456:Gene Flow; D014644:Genetic Variation; D005828:Genetics, Population; D005843:Geography; D060106:Lakes; D010641:Phenotype; D011156:Population Density; D012641:Selection, Genetic; D014337:Trout / Q000235:genetics*; D014867:Water
chemical_list: D014867:Water
keywords: Adaptation; Geometric morphometrics; Intraspecific variation; Lake Superior; Microsatellite DNA; Multiple coinertia analysis; Phenotype