Monitoring reveals two genetically distinct brown trout populations remaining in stable sympatry over 20 years in tiny mountain lakes

Abstract

Detecting population subdivision when apparent barriers to gene flow are lacking is important in evolutionary and conservation biology. Recent research indicates that intraspecific population complexity can be crucial for maintaining a species 0 evolutionary potential, productivity, and ecological role. We monitored the genetic relationships at 14 allozyme loci among *4,000 brown trout (Salmo trutta) collected during 19 years from two small interconnected mountain lakes (0.10 and 0.17 km 2 , respectively) in central Sweden. There were no allele frequency differences between the lakes. However, heterozygote deficiencies within lakes became obvious after a few years of monitoring. Detailed analyses were then carried out without a priori grouping of samples, revealing unexpected differentiation patterns: (i) the same two genetically distinct (F ST C 0.10) populations occur sympatrically at about equal frequencies within both lakes, (ii) the genetic subdivision is not coupled with apparent phenotypical dichotomies, (iii) this cryptic structure remains stable over the two decades monitored, and (iv) the point estimates of effective population size are c. 120 and 190, respectively, indicating that genetic drift is important in this system. A subsample of 382 fish was also analyzed for seven microsatellites. The genetic pattern does not follow that of the allozymes, and in this subsample the presence of multiple populations would have gone undetected if only scoring microsatellites. Sympatric populations may be more common than anticipated, but difficult to detect when individuals cannot be grouped appropriately, or when markers or sample sizes are insufficient to provide adequate statistical power with approaches not requiring prior grouping.

Complex genetic diversity patterns of cryptic, sympatric brown trout (Salmo trutta) populations in tiny mountain lakes

“…In the present case, what appears to be two sympatric brown trout populations have previously been documented to occur in stable sympatry over a minimum of 19 years (almost three generations) in two very small, interconnected mountain lakes in a remote area in the Hotagen Nature Reserve, County of Jämtland, central Sweden (Palmé et al 2013;examining samples collected annually 1987-2005. The two lakes-collectively referred to as Lakes Trollsvattnen-are part of a long-term genetic monitoring study (Jorde and Ryman 1996;Charlier et al 2012;Palmé et al 2013) where the existence of the two genetic clusters was detected when consistent heterozygote deficiency as compared to Hardy-Weinberg proportions became apparent after several years of genetic monitoring using allozyme markers (Jorde and Ryman 1996).…”

“…without visible phenotypic differences and where genetic data have been needed to detect the structures-have rarely been documented. As far as we have been able to find, only two cases in each of the species Arctic char (Wilson et al 2004) and brown trout (Ryman et al 1979;Palmé et al 2013) have been documented, including the one that is the focus of this study.…”

“…The lakes are tiny-0.10 km 2 (ÖT) and 0.17 km 2 (VT), with depths of 1-2 m for the major parts of the area; maximum depths are 5 and 6 m for ÖT and VT, respectively. They are included in a long-term genetic monitoring study, the Lakes Bävervattnen Project, which focuses on spatio-temporal genetic variability patterns in natural populations of brown trout in a pristine, conserved landscape with virtually no direct human manipulation of local fish populations (Jorde and Ryman 1996;Laikre et al 1998;Palm and Ryman 1999;Palm et al 2003;Charlier et al 2011Charlier et al , 2012Palmé et al 2013). The study area is located c. 20 km from the nearest road and is reached only after a full day of hiking in partly harsh terrain.…”

“…The population structures occur in stable sympatry at about the same frequency within both lakes and show no obvious morphological dichotomies (Palmé et al 2013). An extensive screening for phenotypic differences of the two clusters detected only minor tendencies for morphological divergence, and no indication of trophic niche separation (Andersson et al 2016), in spite of a high genetic differentiation (F ST ≈ 0.1; Palmé et al 2013). Differences did occur between lakes, however, indicating the same pattern of plasticity with respect to diet and morphology within both genetic clusters (Andersson et al 2016).…”

“…The two lakes-collectively referred to as Lakes Trollsvattnen-are part of a long-term genetic monitoring study (Jorde and Ryman 1996;Charlier et al 2012;Palmé et al 2013) where the existence of the two genetic clusters was detected when consistent heterozygote deficiency as compared to Hardy-Weinberg proportions became apparent after several years of genetic monitoring using allozyme markers (Jorde and Ryman 1996). The population structures occur in stable sympatry at about the same frequency within both lakes and show no obvious morphological dichotomies (Palmé et al 2013). An extensive screening for phenotypic differences of the two clusters detected only minor tendencies for morphological divergence, and no indication of trophic niche separation (Andersson et al 2016), in spite of a high genetic differentiation (F ST ≈ 0.1; Palmé et al 2013).…”