Adaptive Migratory Divergence Among Sympatiric Brok Charr Populations

Abstract

Ecological processes clearly contribute to population divergence, yet how they interact over complex life cycles remains poorly understood. Notably, the evolutionary consequences of migration between breeding and nonbreeding areas have received limited attention. We provide evidence for a negative association between interpopulation differences in migration (between breeding and feeding areas, as well as within each) and the amount of gene flow (m) among three brook charr (Salvelinus fontinalis) populations inhabiting Mistassini Lake, Quebec, Canada. Individuals (n ϭ 1166) captured throughout lake feeding areas over two consecutive sampling years were genotyped (10 microsatellites) and assigned to one of the three populations. Interpopulation differences in migration were compared based on spatial distribution overlap, habitat selection, migration distance within feeding areas, and morphology. We observed a temporally stable, heterogeneous spatial distribution within feeding areas among populations, with the extent of spatial segregation related to differential habitat selection (represented by littoral zone substrate). Spatial segregation was lowest and gene flow highest (m ϭ 0.015) between two populations breeding in separate lake inflows. Segregation was highest and gene flow was lowest (mean m ϭ 0.007) between inflow populations and a third population breeding in the outflow. Compared to outflow migrants, inflow migrants showed longer migration distances within feeding areas (64-70 km vs. 22 km). After entering natal rivers to breed, inflow migrants also migrated longer distances (35-75 km) and at greater elevations (50-150 m) to breeding areas than outflow migrants (0-15 km; Ϫ10-0 m). Accordingly, inflow migrants were more streamlined with longer caudal regions, traits known to improve swimming efficiency. There was no association between the geographic distance separating population pairs and the amount of gene flow they exchanged. Collectively, our results are consistent with the hypothesis that reduced gene flow between these brook charr populations results from divergent natural selection leading to interpopulation differences in migration. They also illustrate how phenotypic and genetic differentiation may arise over complex migratory life cycles.

Multidisciplinary population monitoring when demographic data are sparse: a case study of remote trout populations

“…Given that (i) CHE N e estimates were largely congruent across methods; (ii) CHE shares a similar life history, migration characteristics, and spatial habitat with PEP (Fraser and Bernatchez 2005); and (iii) CPUE was temporally stable within CHE and PEP, crude estimates of N for these two populations can be formulated based on N e / N ratios reported in other salmonid populations with analogous life histories (mean 0.17, range 0.06–0.31, from Heath et al 2002 and Charlier et al 2011). “Crude” is used here is to reemphasize that the relationship between N e and N is not a strong one, particularly as N e increases, and that salmonid fishes show substantial variation in N e / N ratios (Palstra and Fraser 2012).…”

“…This weak evidence for putatively adaptive population genetic differentiation was unanticipated. First, local adaptation is strongly implicated at the scale between RUP and CHE/PEP based on many phenotypic and life-history differences (Fraser et al 2004, 2005; Fraser and Bernatchez 2005), and on what is known in analogous salmonid populations (Fraser et al 2011). The only outlier SNPs differentiated RUP, reinforcing the previous suggestion that local management should treat RUP and PEP/CHE separately (Fraser et al 2006).…”

“…On the one hand, traditional demographic monitoring is difficult if not impossible in such regions for economic and logistic reasons; sampling can be conducted only intermittently or seasonally (Ferguson and Messier 1997; Berkes 1999; Fraser et al 2006). On the other hand, while sampling of genetic, genomic, and phenotypic metrics is more feasible (e.g., Fraser and Bernatchez 2005; Gomez-Uchida et al 2012), such regions are also more likely to harbor the last remaining population strongholds of the focal species. Thus, changes in these metrics can be more difficult to interpret with respect to moderate or large population demography (Tallmon et al 2012).…”

“…Each river harbors a genetically distinct population comprised of individuals that migrate as juveniles to lake feeding areas for one to four years before returning, predominantly to the same river, to spawn and complete the life cycle (Fraser et al 2004; Fraser and Bernatchez 2005). Each population also contributes differentially to the annual harvest throughout the lake (Fraser and Bernatchez 2005). Yet between 1970 and 2000, Cree fishers anecdotally reported an increase in the average time required to catch a trout and a decrease in the number of trout captured (Fraser et al 2006).…”

Low genetic and phenotypic divergence in a contact zone between freshwater and marine sticklebacks: gene flow constrains adaptation

“…Sex-biased dispersal is often observed in vertebrates, with male- or female biased dispersal resulting from the specific mating systems and/or extent of resource competition [11, 12]. However, phenotypic differences not ascribed to sex may also affect the propensity for dispersal of individuals, such as intraspecific differences in body shape in salmonid fishes adapted to short- or long-distance migration [13, 14]. In the context of clinal patterns, non-random dispersal exhibited by different morphs and ecotypes therefore could have important consequences.…”