Evidence of unidirectional gene flow in a fragmented population of Salmo trutta L

Abstract

Selection, genetic drift, and gene flow affect genetic variation within populations and genetic differences among populations. Both drift and selection tend to decrease variation within populations and increase differences among populations, whereas gene flow increases variation within populations but leads to populations being related. In brown trout (Salmo trutta L.), the most important factor in population fragmentation is disrupted river-segment connectivity. The main goal of the study was to use genetic analysis to estimate the level of gene flow among resident and migratory brown trout in potential hybridization areas located downstream of impassable barriers in one river basin in the southern Baltic Sea region. First, spawning redds were counted in the upper river basin downstream of impassable barriers. Next, samples were collected from juveniles in spawning areas located downstream of barriers and from adults downstream and upstream of barriers. Subsequently, genetic analysis was performed using a panel of 13 microsatellite loci and the Salmo trutta 5 K SNP microarray. The genetic differentiation estimated between the resident form sampled upstream of the barriers and the anadromous specimens downstream of the barriers was high and significant. Analysis revealed that gene flow occurred between the two forms in the hybridization zone investigated and that isolated resident specimens shared spawning grounds with sea trout downstream of the barriers. The brown trout population from the river system investigated was slightly, internally diversified in the area accessible to migration. Simultaneously, the isolated part of the population was very different from that in the rest of the basin. The spawning areas of the anadromous form located downstream of the barriers were in a hybridization zone and gene flow was confirmed to be unidirectional. Although they constituted a small percentage, the genotypes typical upstream of the barriers were admixed downstream of them. The lack of genotypes noted upstream of the barriers among adult anadromous individuals might indicate that migrants of upstream origin and hybrids preferred residency.

Figure 1

Parsęta River basin with migration barriers, potential hybridization zones and sampling sites (created by author in ArcMap 10.7.1).

Figure 2

Allelic patterns and heterozygosity in brown trout from the Parsęta River basin sampled in 2017 (left) and 2018 (right). MNA mean allele number in the population, HO observed heterozygosity, HE expected heterozygosity, AR allelic richness PAR private allele richness, Sea dist distance to the sea (km). Red dots show location of impassable barriers.

Figure 3

Clustering of brown trout collected in the Parsęta basin in 2017 (upper bar) and 2018 (lower bar), K = 2. Each individual is represented by a column divided into K shades with each shade representing membership of a particular cluster (created by author in ArcMap 10.7.1).

Figure 4

Average membership coefficient q from five independent runs for K = 2 calculated for 50 juveniles sampled in the potential hybridization zone in 2017. Green bars represent clade 1 (upstream of the barriers) and blue bars represent clade 2 (area of free migration).

Figure 5

Average membership coefficient q from five independent runs for K = 2 calculated for 36 juveniles sampled in the potential hybridization zone in 2018. Green bars represent clade 1 (upstream of barriers) and blue bars represent clade 2 (area of free migration).

Figure 6

Results of the pairwise assignment test calculated as the difference between log likelihood of assignment to the 17MU and 17MDH locations for 50 juveniles from the hybridization zone in 2017. Specimens with negative values were more strongly related to the population downstream of the barrier. Green bars = F0 migrants, orange bars = hybrids, blue bars = anadromous genotypes.

Figure 7

Results of the pairwise assignment test calculated as the difference between the log likelihood of assignment to 18MU and 18MDH locations for 36 juveniles from the hybridization zone in 2018. Specimens with negative values were more strongly related to the population downstream of the barrier. Green bars = F0 migrants, orange bars = hybrids, blue bars = anadromous genotypes.

Figure 8

Average membership coefficient q from five independent runs for K = 2 calculated for 43 juveniles sampled in the potential hybridization zone in 2017 and genotyped with an SNP microarray. Green bars represent clade 1 (upstream of barriers) and blue bars represent clade 2 (area of free migration).

Figure 9

Average membership coefficient q from five independent runs for K = 2 calculated for 24 juveniles sampled in the potential hybridization zone in 2018 and genotyped with an SNP microarray. Green bars represent clade 1 (upstream of barriers) and blue bars represent clade 2 (area of free migration).