Hierarchical population structure and genetic diversity of lake trout (Salvelinus namaycush) in a dendritic system in Northern Labrador

Abstract

Summary Freshwater systems are ideal for landscape genetics studies; they generally exhibit asymmetry in gene flow with populations arranged in a hierarchical, dendritic fashion. Such gene flow asymmetry has the potential to influence the distribution of genetic variation, with downstream populations typically exhibiting higher genetic diversity than headwater populations. Lake trout (Salvelinus namaycush) are widely distributed throughout temperate North America. We examined the relationship between landscape attributes, molecular genetic diversity and differentiation among lake trout populations inhabiting a hierarchically structured freshwater system in northern Labrador (the Kogaluk River drainage). Lake trout (N = 567) collected from ten lakes in this drainage were examined for polymorphism at 12 microsatellite loci. The lakes differed in connectivity, size, elevation and position within the Kogaluk catchment. We observed relatively high levels of population structure and little migration among lakes. Waterfalls were identified as a factor contributing to the observed differentiation. Although a preliminary analysis suggested an isolation‐by‐distance pattern, a subsequent decomposed pairwise regression analysis allowed the identification of outlier lakes. Removal of these outlier lakes and of one other headwater lake, isolated from the rest by several waterfalls, led to the disappearance of the isolation‐by‐distance pattern, suggesting very little or no migration among lakes despite migration being physically possible. Estimates of effective population size correlated with lake area but did not differ between headwater and downstream populations, nor was there a relationship with elevation, a result consistent with the evidence of very little to zero gene flow among lakes. Our study is the first to describe patterns of genetic diversity among lake trout populations inhabiting a spatially fragmented system in an ecologically pristine and sensitive area of northern Canada, the barren grounds of northern Labrador.

Loss of genetic integrity in wild lake trout populations following stocking: insights from an exhaustive study of 72 lakes from Québec, Canada

“…These results are concordant with those reported by McCracken et al. () in lake trout as well in other nonmigratory salmonids (Koskinen et al. ; Ozerov et al.…”

“…), as is the effective population size ( N E ; this study and McCracken et al. ), variation in population size and N E among lakes probably influences the extent of population genetic diversity through variable intensity of genetic drift (Charlesworth ; Frankham et al. ).…”

“…; McCracken et al. ) as well as with values observed in other nonmigratory salmonids (global F ST brook charr 0.23, Marie et al. ; bull trout 0.33, Taylor et al.…”

“…Other studies conducted on lake trout also documented little or no evidence of geographical grouping (Halbisen ; McCracken et al. ). Thus, each unstocked lake trout population could be considered as a unique ‘genetic entity’ without any clear regional grouping.…”

Life-history characteristics and landscape attributes as drivers of genetic variation, gene flow, and fine-scale population structure in northern Dolly Varden (Salvelinus malma malma) in Canada

“…Each of these clusters was characterized by significant intragroup spatial structure that was essentially parsed down to river system and (or) life-history variants (isolated populations) within a watershed. Hierarchical genetic population structure has been described in a wide range of fishes (Vähä et al 2007;Dionne et al 2008;Cook et al 2011) and is typically associated with drainage-or watershedlevel groupings over larger spatial scales (Warnock et al 2010;McCracken et al 2013) or structured into regions of limited dispersal or potential barriers to gene flow . Overall, our data are concordant with these results and support the idea that proximate populations sampled within the same relative region over larger geographic scales are likely connected by some degree of gene flow (be it contemporary or historical) that promotes hierarchal population genetic structure.…”